chore(developer): verify kvks files and report errors

Fixes #8886.

.kvks compiler now returns helpful errors for xml parse failures and
schema validation errors, and for invalid virtual key codes. Introduces
extra infrastructure for reporting and unit testing messages to kmc-kmn.

This also fixes unhandled xml load exceptions and simplifies the error
reporting coming out of the kvks loader, on the basis that only one type
of error was being reported anyway.