Improve errors & warnings

[hendrikvanantwerpen]

Currently we only have errors, and we always return on the first error. This is annoying, every error requires an additional run to discover the following. Most errors are local to a stanza, and we could easily discover multiple errors in one run.

Improvements for error handling:

• Collect errors (at least during checking) and return all of them, instead of only the first.
• Introduce warnings and return those even for successful parses.