Fail One File Instead of the Whole Run on a Per-File Exception #1069

Behavior

Neither Process.ProcessFile() nor the per-file loop in ProcessDriver.ProcessFiles() catches an exception. The only catch is around the whole PLINQ ForAll. An exception in any step, for example an IOException from a File.Move or File.Delete in a remux step, ends the run for every remaining file, and the command exits with 1.

Reproduction shape

1. Process a folder holding several files.
2. Make one step throw for one file, for example by holding an exclusive lock on its .tmp2 remux target on Windows.
3. The run stops at that file, and later files are not processed.

Expected

The exception fails that one file, records it as an error with its failing operation, and the run continues.

Found while drawing the process command diagrams, ARCHITECTURE.md "Process Command Logic", suspect branch S2.