Add test for each crate::Error variant that can be reproduced.

[marcospb19]

This is important before a major release. Just to make sure that the errors are being correctly detected, from the CLI all the way to the conflict in output files, or folders.

The API for commands::run is already correctly exposed for testing, here's an example:

https://github.com/vrmiguel/ouch/blob/master/tests/compress_and_decompress.rs#L93-L97