Provide error details in auth error dialog if Canvas developer secret is wrong

If an LMS app installation is configured with an incorrect developer secret, the Canvas authorization process fails without a useful error message:

1. Launch the assignment in the "Wrong Developer Secret" course at https://hypothesis.instructure.com/courses/120/assignments/848
2. Click the "Authorize" button when prompted during the assignment launch
3. Click "Authorize" in the popup dialog

Result:

Clicking on the "email support" link opens an email with a blank body.

Ideally this dialog should include some hint as to what exactly went wrong. If Canvas doesn't provide us with that information then perhaps we can still include something useful to narrow the range of possibilities.