Improve returned user errors on RDAT submission

[tkaragianes]

The server goes through a number of steps (tagging, heatmap generation, thumbnail creation) when a new RDAT is uploaded. If any of these steps errors (even if the overall upload succeeds), the user is presented with a 500 error that doesn't provided any more context. We ought to surface the steps to the user more concretely, and be clear if the upload fails (e.g. no save of the rdat) vs a processing step failed but data upload succeeded.

Specifically, this was surfaced with RMDB entry RNAPZ14_STD_0003, which uploaded correctly, but failed to generate a secondary structure thumbnail via VARNA:

CalledProcessError: Command 'java -cp /home/ubuntu/Server_RMDB/data/VARNA.jar ...  -o /home/ubuntu/Server_RMDB/data/thumbnail/RNAPZ14_STD_0002-0.png' returned non-zero exit status 1, referer: https://rmdb.stanford.edu/deposit/submit/