[frontend/submission_manager] catch DocumentTooLarge exception

[anthonygego]

Fixes #697.

This is a first mitigation and in all case useful, as the limit is set on the total command sent to the MongoDB server, which cannot be computed before.

Do not update user stats in case of error.

Runfile to test :

  #!/bin/bash
  dd if=/dev/random of=test.out bs=4M count=100
  cat test.out | tr -dc '[:alpha:]' | feedback-msg -a
  feedback-result success

[Drumor]

LGTM