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 
Why the semi colon?