Raise informative error if subject's Freesurfer derivatives do not exist when using `--fs-no-resume`

[tsalo]

Is your feature request related to a problem? Please describe.
I am working on copying over session-wise anatomical processing fMRIPrep to ASLPrep, and I noticed that some of my tests were failing with the following error message:

ERROR    nipype.workflow:tools.py:51 Node midthickness failed to run on host 46fd79a18ca1.
ERROR    nipype.workflow:tools.py:68 Saving crash info to /src/aslprep/.circleci/out/test_003_full/aslprep/sub-01/ses-1/log/20251029-163525_92d85f56-bcc3-419c-a427-bcb62247a866/crash-20251029-163633-root-midthickness-3c667ffd-8e51-4ecb-b65b-0904b32f018a.txt
Traceback (most recent call last):
  File "/opt/conda/envs/aslprep/lib/python3.12/site-packages/nipype/pipeline/plugins/multiproc.py", line 323, in _send_procs_to_workers
    num_subnodes = self.procs[jobid].num_subnodes()
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/envs/aslprep/lib/python3.12/site-packages/nipype/pipeline/engine/nodes.py", line 1312, in num_subnodes
    self._check_iterfield()
  File "/opt/conda/envs/aslprep/lib/python3.12/site-packages/nipype/pipeline/engine/nodes.py", line 1335, in _check_iterfield
    raise ValueError(
ValueError: Input in_file was not set but it is listed in iterfields.

After some digging, I realized that the error was stemming from the fact that I was passing in Freesurfer derivatives with just the subject ID, but now sMRIPrep is looking for ones with the subject and session. Unfortunately, the error message isn't very clear.

Describe the solution you'd like
A more informative error message - e.g., "No Freesurfer derivatives found at <path to subject's expected derivatives>" - would be great.

Describe alternatives you've considered
None

Additional context
n/a