Revert "Override default petsc4py error handler"

This reverts commit c0107bc5cc0814b26ed31d3bf3e04e742c66b54b.

The mpiabort error handler turns out to be more trouble than it's worth,
since it hides any useful stack traces we might see.  Having found the
cause of the bug that this change was originally made to catch, we can
safely revert it.