Improve Error Information when a duplicate link for a listing will be created

[fsantiago]

We have a lot of listings that might have the same name when we link the Canvas course to Catalog. Yesterday, I spent hours trying to get a listing tied and saved into Catalog and I only received a console message saying that there was an error and the listing could not be saved. I checked all the settings, my access to the catalog, I tried saving other listings with no issues until I tried and typed a different listing link. Then the program let me save.

Please improve the error messaging so we can know what the issue is.

Thanks