Use and report errors that inflate(3) can return, instead of

Use and report errors that inflate(3) can return, instead of
aborting silently, and pretending everything went alright.
This lets gzip(1) (especially gzip -t) detect truncated archives
and curruptions that violate the structure of the zlib format.
Unquiet compress(1) in zless/zmore, so the user has a chance
to see errors on corrupted archives.
OK millert@, markus@