Error message for non-existing config-file is suboptimal

[maxnoe]

Describe the bug
Due to the new support for multiple config files, the error message for non-existing files is a bit verbose / confusing:

❯  ctapipe-process --config this/does/not/exist
2022-04-19 11:08:18,094 CRITICAL [ctapipe.ctapipe-process] (application.inner): Bad config encountered during initialization: The 'config_files' trait of a ProcessorTool instance contains  which expected <traitlets.traitlets.List object at 0x7f395f9dcd30>, not the str 'Path "/home/maxnoe/this/does/not/exist" does not exist'.

Expected behavior

Clear error message like configuration file "<file>" does not exist

[kosack]

I guess this is because the config_files Path trait fails first, as it checks the existence before any config loading happens. We could make a sub-class called ConfigPath perhaps that outputs a more useful error message?

[kosack]

I guess it's also the fact that this is a List(Path) trait, not just Path - not sure why it complains that it is not a List though, since that should not be a problem as it should accept it as a single-item list.

[maxnoe]

Just ran into this again, really, really misleading and annoying. Need to fix

[maxnoe]

from ctapipe.core import Tool, run_tool


class TestTool(Tool):

    def setup(self):
        pass

    def start(self):
        print(self.config_files)



if __name__ == '__main__':
    run_tool(TestTool())

results in:

❯ python test_tool_config.py -c i_do_not_exist.yaml
[]

no error, just an empty config.

[maxnoe]

I have a hard time debugging this, since it never actually seems to enter ctapipe code, the values seem to be lost before in traitlets (e.g. Path.validate is never called on the given path)

[kosack]

once again #1406 ....