Update student training messaging in authoring.

[ORA-731](https://openedx.atlassian.net/browse/ORA-731)

This reverts the change I made earlier this week to wrap all server-side student training errors as "Student training example option does not match the rubric."  This message is confusing when validation fails because a criterion has 0 options.

@stephensanchez 
The Studio validation error box doesn't show newlines correctly.  Assuming that client-side validation is working correctly, we shouldn't have multiple errors returned from the API at all, so in most cases it will just display the single error message.