Improve Python Task Errors

[robin-173]

Review the overall errors from Python tasks. We need to be able to differentiate easily errors which are caused to SAYN setup (e.g. missing module file, wrong name of module file) or Python code error. List of things to fix:

• wrong reference to class in task definition leads to fairly generic error message (e.g. No module named 'sayn_python_tasks.test') -> capture when the file does not exists (bear in mind the class could be defined at multiple levels: Test, test.Test, test.test.Test).
• prettify the missing init.py error
• make the error message clearer when missing Result object returned (i.e. return self.success()/self.fail()). The current error message is: ✖ Result.Err (task_result::missing_result_error): {'result': None}
• review overall python errors