Abort if cannot find config

[benjimin]

If TCRM cannot find the config file specified (via shell cmdline args) by the user, as is prone to occur due to TCRM interpreting paths differently than the parent shell, then it continues and executes some processes in accordance with defaults, which is misleading to debug.

Suggested actions:

• TCRM should abort with error, if explicit specifications by user cannot be followed (e.g. if standard library ConfigParser.read returns empty).
• Should use importlib.resources (preferred) or pkg_resources rather than modifying working directory, and certainly should not do latter before finished processing command line arguments. (Dovetails with using standard install methods, perhaps in develop mode, instead of customising shell environment.)