Improve errors in case of invalid yaml

[miniBill]

This PR does two things:

1. always tries to parse a file as both json and yaml (favouring the most probable one)
2. gives the error message coming from the most probable format

This means that yaml files mostly get yaml errors, and json file json errors, but if we guess wrong we still use both parsers.

[miniBill]

This fixes #40

[github-actions]