rm error silencing, fail fast if no yaml lib exists

[tomwojcik]

Fixes #29

• drop global error silencing on import / during init eval
• fail fast if required dependencies are missing

[NowanIlfideme]

Tested locally, this works and gives the following readable error:

...
ModuleNotFoundError: No module named 'yaml'
...
ImportError: Could not import ruamel.yaml or pyyaml, please install at least one of them.

Will just need to check the mypy errors failing in the PR 😃

Honestly, I don't quite remember why install-without-extras was ignored. I think it was a versioning-related problem, which was fixed by switching to bump2version (but I forgot to revert the try-except block).

[NowanIlfideme]

Thank you for noticing this, and sorry for a slow response and merge - busy New Year's. :)