Hanged when submitting a new external source item

[paulo-graca]

Describe the bug

When a 403 error occurred the system got hanged.

To Reproduce

Steps to reproduce the behavior:

1. Using DSpace 7.6.1
2. The following error must occur - Access denied when importing content of external source for a valid submitter DSpace#9797
3. The the system hangs like in this image, giving the notion that is still processing
   

Expected behavior

The user should be notified about the occurred error, not being hanged nor have the feeling he needs to wait until it ends.

Related work

DSpace/DSpace#9797

[tdonohue]

While this appears to be caused by DSpace/DSpace#9797, I agree that the submission form should have better error handling. Ideally the form shouldn't hang if an error occurs, but should display some information about the error and then allow the user to proceed (or not). Needs a volunteer.