Make validator messages language-independent and get rid of i18n

[vergeev]

I propose making validator error messages serve one purpose: indicating where the problem is. In other words, they should contain file and variable names, line numbers, error counts, but should not contain explicit error descriptions.

More explicit error descriptions belong to the documentation and, when needed, can be restored from the error slug by the client.

So for example the message "%s class is too broad; use a more specific exception type" can be restored by the client himself by looking at the error slug.

And messages like "for example, line number %s should be replaced by language-independent constructions like "filename:line_number".

So the output

    >>> from fiasko_bro import validate
    >>> validate('/path/to/folder/')
    [('camel_case_vars', 'for example, rename the following: WorkBook'), ('file_too_long', 'coursera.py')]

will turn into

    >>> from fiasko_bro import validate
    >>> validate('/path/to/folder/')
    [('camel_case_vars', 'WorkBook'), ('file_too_long', 'coursera.py')]