Need better error for missing file in pytmc template

[ZLLentz]

If your PLC project repo is missing a file (any file, even one not related to the ioc build), the pytmc template command fails with a confusing error:

Traceback (most recent call last):
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/bin/pytmc", line 10, in <module>
    sys.exit(main())
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/bin/pytmc.py", line 106, in main
    func(**kwargs)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/bin/template.py", line 830, in main
    raise stashed_exception
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/bin/template.py", line 821, in main
    template_args.update(projects_to_dict(*projects))
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/bin/template.py", line 268, in projects_to_dict
    for key, value in project_to_dict(path).items():
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/bin/template.py", line 219, in project_to_dict
    projects = {fn: parser.parse(fn) for fn in project_files}
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/bin/template.py", line 219, in <dictcomp>
    projects = {fn: parser.parse(fn) for fn in project_files}
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 51, in parse
    return TwincatItem.parse(root, filename=fn, parent=parent)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 407, in parse
    return cls(element, parent=parent, filename=filename, name=name)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 224, in __init__
    self._add_children(element)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 300, in _add_children
    self._add_child(child_element)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 309, in _add_child
    child = self.parse(element, parent=self, filename=self.filename)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 407, in parse
    return cls(element, parent=parent, filename=filename, name=name)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 224, in __init__
    self._add_children(element)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 300, in _add_children
    self._add_child(child_element)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 309, in _add_child
    child = self.parse(element, parent=self, filename=self.filename)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 407, in parse
    return cls(element, parent=parent, filename=filename, name=name)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 224, in __init__
    self._add_children(element)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 2134, in _add_children
    self._finish_lazy_loading(self._xti_files)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 347, in _finish_lazy_loading
    self._children[idx] = child.load(file_map)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 455, in load
    obj._finish_lazy_loading(file_map)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 349, in _finish_lazy_loading
    child._finish_lazy_loading(file_map)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 347, in _finish_lazy_loading
    self._children[idx] = child.load(file_map)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 455, in load
    obj._finish_lazy_loading(file_map)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 349, in _finish_lazy_loading
    child._finish_lazy_loading(file_map)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 347, in _finish_lazy_loading
    self._children[idx] = child.load(file_map)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.8.1/lib/python3.9/site-packages/pytmc/parser.py", line 453, in load
    info = file_map[self.key]
KeyError: (<class ‘pytmc.parser.Box’>, ‘15’, ‘ffo.xti’)

It's good that this gets caught in some capacity, but the error should be made more clear, e.g. something like:
"RuntimeError: file named {filename} missing from the repo!"

We can probably handle this by catching a KeyError in this line:

pytmc/pytmc/parser.py

Line 453 in 2c4f412

info = file_map[self.key]