refactoring check error reporting

[jplitza]

The current setup for the check script is a bit difficult and verbose. For example, having to count the errors manually is a PITA, introducing warnings would make the issue even worse.

Thus, I would like to

• introduce functions or classes for different types of messages during the validation process (errors, warnings, ...) that count the number of occurrences of their type
  • make the whole thing a class Validator, instead of the function do_checks, so we still could invoke it multiple times with different srcdir and get different counters
• make the error messages more structured, like [community] bgp.peer1.ipv4 invalid address: 123.456.789.0 or [community1,community2] networks.ipv4 overlap: 0.0.0.0/0 (community1), 10.0.0.0/24 (community2). I'm not totally sure on the details, but I find the current free text hard to correlate to the YAML.

This pull request is currently there to discuss these ideas. I'd like to hear comments. :)