Prefix commands should display more complete error stack in verbose mode

The prefix commands (e.g. submit, parallel) only display the last error message for readability and user-friendliness. Yet while debugging, we often need the whole error stack and end up commenting out by hand some lines at the end of the file and often forget undoing this dirty hack when pushing. It would be better to display the full error stack whenever running in verbose mode. @lmoureau was suggesting to use import traceback.