500 error blocks import of all uploads

Web UI S3 upload does not handle 500 errors on complete, which also blocks the user from completing an import.

Steps to reproduce following HF33 in https://nxdev-uat-10-10.beta.nuxeocloud.com/

1) Create a long-running import.  The longer the content upload, the easier it seems to reproduce.  I reproduced multiple times by trying to upload 200, 50 MB pictures which takes over 4 hours with 5 Mbps upload speed.   

2) Wait for one file/batch to encounter a 500 error on complete.  Observe in the UI that the upload did not finish and is now "stuck" in an incomplete state.

3) User is blocked from continuing.  Cannot import after removing the file that is stuck, or assign properties and complete the import.  The only option is for the user to cancel the entire 200 file import.

Expectation is that when a file fails to complete, there is a visual indicator in the UI, and the ability to finish the import of the completed files.