validators should have a default message level and be overrideable in config

[aelkiss]

• default message level can either be warning or error
• what to do about validators that can emit either warnings or errors? If specified level in config is:
  • unspecified/default: warnings are warnings & errors are errors
  • warning: demote errors to warnings
  • error: promote warnings to errors.

This approach should work for all validators and should only require changing the create_error and create_warning methods.

[aelkiss]

Migrated to HTREPO-144