FS API errors on missing volume

[zebulasampedro]

When retrieving details for a missing volume, the FS API produces the following 500 error:

Traceback (most recent call last):
  File "/opt/sandstone-project/venv-sandstone/lib/python2.7/site-packages/tornado/web.py", line 1443, in _execute
    result = method(*self.path_args, **self.path_kwargs)
  File "/opt/sandstone-project/venv-sandstone/lib/python2.7/site-packages/sandstone/lib/decorators.py", line 21, in wrapper
    return method(self, *args, **kwargs)
  File "/opt/sandstone-project/venv-sandstone/lib/python2.7/site-packages/sandstone/lib/filesystem/handlers.py", line 72, in get
    res = self.fs.get_filesystem_details()
  File "/opt/sandstone-project/venv-sandstone/lib/python2.7/site-packages/sandstone/lib/filesystem/interfaces.py", line 42, in get_filesystem_details
    size, used, avail, used_pct = out.split()[-5:-1]
ValueError: need more than 0 values to unpack
ERROR:tornado.access:500 GET /user/vagrant/a/filesystem/ (127.0.0.1) 14.50ms

A dismissable warning should instead be rendered, indicating that the specified volume cannot be found.