[BUG] MultiProc crashes are not correctly handled

[oesteban]

Summary

Since the latest refactor, MultiProc calls self._report_crash with a traceback argument and such function is not defined:

File "/usr/local/miniconda/lib/python3.6/site-packages/niworkflows/nipype/pipeline/engine/workflows.py", line 591, in run
runner.run(execgraph, updatehash=updatehash, config=self.config)
File "/usr/local/miniconda/lib/python3.6/site-packages/niworkflows/nipype/pipeline/plugins/base.py", line 182, in run
self._send_procs_to_workers(updatehash=updatehash, graph=graph)
File "/usr/local/miniconda/lib/python3.6/site-packages/niworkflows/nipype/pipeline/plugins/multiproc.py", line 250, in _send_procs_to_workers
self._report_crash(self.procs[jobid], traceback=traceback)
TypeError: _report_crash() got an unexpected keyword argument 'traceback'

I'll propose a fix today.