seq: improve quality of format-checking code

seq: improve quality of format-checking code

* src/seq.c (validate_format): Remove. Migrate its checks into...
(long_double_format): Report an error and exit if an error is found,
instead of returning NULL.  All callers changed.
Use a more-consistent format for diagnostics.
* tests/misc/seq: Adjust to the more-consistent format for diagnostics.