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The existing behaviour for seq-subseq is to error when indexes are too
large, but to silently ignore numbers which are too negative for lists.
String and vector handling errors in both cases. This has been
regularlised.

Error signalling behaviour has been explicitly added to the docstring of
seq-subseq, and also to cl-subseq which largely defers to
seq-subseq (and is therefore also impacted by this change).

Tests have been added for these exceptional cases, as well as one non
exceptional base case.