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 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 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: (, ‘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: https://github.com/pcdshub/pytmc/blob/2c4f4121cd109d2d2cee9bdcc357d792ddca01f7/pytmc/parser.py#L453