Improve error messages in ReduceASLFiles

[tsalo]

Closes none, but should make the error messages that could be raised by ReduceASLFiles more informative.

I noticed some related errors in the Sentry logs for this week.

Changes proposed:

• Improve error messages.
• Disable SyN SDC for test_001, which doesn't have the necessary metadata.
• Pin SDCFlows to 2.6.0 (closes #354).

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (d6c868c) 78.98% compared to head (780cfcd) 78.90%.

Files	Patch %	Lines
aslprep/interfaces/utility.py	50.00%	1 Missing and 1 partial :warning:
aslprep/utils/asl.py	50.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #379 +/- ## ========================================== - Coverage 78.98% 78.90% -0.08% ========================================== Files 39 39 Lines 4149 4153 +4 Branches 602 604 +2 ========================================== Hits 3277 3277 - Misses 676 678 +2 - Partials 196 198 +2 ```

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[tsalo]

test_001 may not have the necessary metadata for SyN:

Traceback:
        Traceback (most recent call last):
          File "/usr/local/miniconda/lib/python3.10/site-packages/nipype/interfaces/base/core.py", line 397, in run
            runtime = self._run_interface(runtime)
          File "/usr/local/miniconda/lib/python3.10/site-packages/sdcflows/interfaces/epi.py", line 57, in _run_interface
            self._results["readout_time"] = get_trt(
          File "/usr/local/miniconda/lib/python3.10/site-packages/sdcflows/utils/epimanip.py", line 228, in get_trt
            raise ValueError("Unknown total-readout time specification")
        ValueError: Unknown total-readout time specification