I do not see an API call to get the error report.
I do have a faint memory, from when I first started working on Canvas... I had a conversation with support asking for info related to the error_report_id and it seems like they were able to retrive this info using a url.
At the time I was trying to figure things out, and did not make a note.
You might try contacting support to see if they can share some info with you.
My experience with the API and error_report_id so far has been related to syntax or incorrect parameters.
An "error report" has not helped me over a hump yet, but that doesn't mean it wouldn'e shed some light.
Please post back if you learn anything.