Add error checking for missing oidc redirect and launch endpoint if LTI 1.3 launches are selected

Follow the instructions to set up an LTI Advantage tool in Sakai:  https://github.com/sakaiproject/sakai/blob/master/basiclti/basiclti-docs/resources/docs/Advantage_TestPlan.xls Make various mistakes putting bad and / or blank values in the LTI13 fields and attempt launches and make sure the error messages at least lead to an understanding of what is wrong instead of 500 errors or tracebacks. For example leave the OIDC Initiation endpoint blank and leave the redirect end point blank.   There should be an error message that tells what is wrong instead of being cryptic.