An error should be thrown when LTI launch using incorrect assignment ID (PRB0045585)

[xcompass]

If instructor accidently put in incorrect assignment ID in the LTI launch URL, there should an error showing when people are launching LTI using the URL. Otherwise, the grade sync may not work correctly.

Currently, it checks if the assignment exists but do not throw errors.

Related code: https://github.com/ubc/compair/blob/master/compair/models/lti_models/lti_resource_link.py#L41