Proper error handling for missing config files

[mbonish]

Description
When there is missing user-sync-config.yml or siggn-config.yml the errors are not being caught, but instead a stack trace is exposed. like as follows:

2021-11-01 18:22:42 17608 INFO sign_config - Using main config file: sign-sync-config.yml (encoding utf8)
Traceback (most recent call last):
File "C:\Users\adorton\projects\user-sync.py\venv\Scripts\user_sync-script.py", line 33, in
sys.exit(load_entry_point('user-sync', 'console_scripts', 'user_sync')())
File "c:\users\adorton\projects\user-sync.py\venv\lib\site-packages\click\core.py", line 1128, in call
return self.main(*args, **kwargs)
File "c:\users\adorton\projects\user-sync.py\venv\lib\site-packages\click\core.py", line 1053, in main
rv = self.invoke(ctx)
File "c:\users\adorton\projects\user-sync.py\venv\lib\site-packages\click\core.py", line 1659, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "c:\users\adorton\projects\user-sync.py\venv\lib\site-packages\click\core.py", line 1395, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "c:\users\adorton\projects\user-sync.py\venv\lib\site-packages\click\core.py", line 754, in invoke
return __callback(*args, **kwargs)
File "c:\users\adorton\projects\user-sync.py\user_sync\app.py", line 205, in sign_sync
run_sync(sign_config.SignConfigLoader(kwargs), begin_work_sign)
File "c:\users\adorton\projects\user-sync.py\user_sync\config\sign_sync.py", line 89, in init
self.raw_config = self._load_raw_config(filename, encoding)
File "c:\users\adorton\projects\user-sync.py\user_sync\config\sign_sync.py", line 134, in _load_raw_config
return self.config_loader.load_root_config(filename)
File "c:\users\adorton\projects\user-sync.py\user_sync\config\common.py", line 341, in load_root_config
return self.load_from_yaml(filename, self.root_path_keys)
File "c:\users\adorton\projects\user-sync.py\user_sync\config\common.py", line 377, in load_from_yaml
raise AssertionException('No such configuration file: {}'.format(filepath))
user_sync.error.AssertionException: No such configuration file: C:\Users\adorton\projects\user-sync.py\sign-sync-config.yml

Steps to reproduce

• sign-sync config.yml and/or user-sync-config.yml (missing)
• Command-line options used: Sign_sync and sync

Expected behavior
Error should be handled with critical logger.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment

• UST version: 2.7.0rc3 (sign_phase_2; DME)
• OS type and version: Windows