Parser: improve error message handling

[chqrlie]

• use single on_error handler with error level and message arguments
• remove Warning token type, never returned anyway.
• improve #error and #warning message parsing consistency
• make num_error messages non fatal
• fix #warning behavior

[bvdberg]

It seems to make parsing slower... What was it that you were trying to improve?

[chqrlie]

It seems to make parsing slower...

This is surprising, the changes should not affect the inner loops, only the error cases.

What was it that you were trying to improve?

I was just trying to improve the consistency:

• both warnings and errors should be produced synchronously,
• #warning used to output an error with on_warning, itself using diag.error instead of diag.warn
• num_error are errors, not warnings, but non fatal.
• #error returned an error token without showing the error message.