ENH: Allow --use-syn-sdc to take a "warn" option to avoid exiting when PE dir is unavailable

[effigies]

The main use case here is when writing a script to run fMRIPrep on heterogeneous datasets. This avoids having to pre-check each dataset/subject for whether PhaseEncodingDirection is available; you just run it if you can.

Also simplifies the (probably vanishingly rare) case where PhaseEncodingDirection may be available for some tasks/runs within a subject but not others.

The old behaviors are preserved, so this is bug-fix safe. --use-syn-sdc and --use-syn-sdc error are synonyms. --use-syn-sdc warn is the new feature.

Happy to consider alternative approaches, but I think this is useful functionality.

[mgxd]

random thought - when do we plan on removing the "EXPERMENTAL" tag?

[effigies]

When there's a ground truth. :-P

IDK. Maybe let's discuss at next bimonthly?

[mgxd]

tests are now failing...

Process Process-2:
Traceback (most recent call last):
  File "/opt/conda/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/opt/conda/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/conda/lib/python3.8/site-packages/fmriprep/cli/workflow.py", line 118, in build_workflow
    retval["workflow"] = init_fmriprep_wf()
  File "/opt/conda/lib/python3.8/site-packages/fmriprep/workflows/base.py", line 85, in init_fmriprep_wf
    single_subject_wf = init_single_subject_wf(subject_id)
  File "/opt/conda/lib/python3.8/site-packages/fmriprep/workflows/base.py", line 337, in init_single_subject_wf
    raise ValueError(message)
ValueError: Fieldmap-less (SyN) estimation was requested, but PhaseEncodingDirection information appears to be absent.
fMRIPrep: Please report errors to https://github.com/nipreps/fmriprep/issues

[effigies]

Completely failed to look at the test failures. That's really weird, though...