MPI cleanup removes useful traceback from output

[msyriac]

These lines: https://github.com/simonsobs/pixell/blob/d3167f935f9a9c147fe698e1eb4e83d3a9daeb39/pixell/mpi.py#L23

when not commented cause the MPI output to not print the traceback for the error that caused the script to crash (if any), which leads to a lot of wasted time. I can reproduce this on niagara with both Intel MPI and OpenMPI.