Fix backup and exit code issues

- Set GMX_MAXBACKUP to -1 (if not set by user) so that tests
  don't fail after being run 99 times
- Check the exit value of grompp
- Don't rerun mdrun if mdrun failed with any error message
  other than those explicitly handeled by how_should_we_rerun_mdrun
  for parallelization reasons

Fixes #1455

Change-Id: I5b4d2204d7199cff521f4400143a80b1fab67e5c