Reformat error messages

Now every error message produced by Git::Hooks follow this pattern:

  [MODULE] TITLE

    BODY

<MODULE> is the name of the module where the error was detected. Usually
the name of a plugin module.

<TITLE> is a single line description of the error.

<BODY> is optional. It can carry extra details about the error. There is
always an empty line before and after it, and all of its lines are
prefixed by at least two spaces.