Better error handling in PoFile parsing

I needed some better errors class for some exceptions during the PoFile parsing.

In the meanwhile, [NoMethodFound](https://github.com/haeky/get_pomo/compare/grosser:master...haeky:errors#diff-4ad0782e8c7e91f7b85e4351bf96e001L109) was actually never hit because of the [RegExp] (https://github.com/haeky/get_pomo/blob/2e7926dba32344edcd25056fb5d57f81e795da78/lib/get_pomo/po_file.rb#L103) in `method_call?`. I've moved the method elsewhere to better represent the exception.