The `error_expected` custom attribute used to contain
both the class name and the error message. This had
the following issues:

* Combining data in the same custom attribute limits
the ability to query.
* The additional error class and message data is only
needed for ignored errors, since this data isn't
available elsewhere.

The following changes were made:
* `error_expected` will always have the value True
if present.
* `error_ignored` no longer exists.
* `error_ignored_class` will contain the error module
and class for ignored errors.
* `error_ignored_message` will contain the error message
for ignored errors.

ARCHBOM-1708