Improve CLI error message

[jlorieau]

Improve CLI error messages when checks files format errors are encountered

[jlorieau]

For example, passing a toml file with the incorrect .yaml extension produces an Exception:

Traceback (most recent call last):
...
yaml.parser.ParserError: while parsing a block mapping
  in "test.yaml", line 1, column 1
expected <block end>, but found ','
  in "test.yaml", line 1, column 48