install: errors for a target that is not a directory differ from GNU

[abendrothj]

When the destination of install is not a usable directory, the error wording differs from GNU (uutils at 2b85f6f, GNU 9.12 built from source):

command	GNU	uutils
install f g regular	target 'regular': Not a directory	target 'regular' is not a directory
install f g missing	target 'missing': No such file or directory	target 'missing' is not a directory
install f regular/	cannot stat 'regular/': Not a directory	target 'regular/' is not a directory
install f missing/	cannot create regular file 'missing/': Not a directory	target 'missing/' is not a directory
install -D -t regular/sub f	failed to access 'regular/sub': Not a directory	cannot create directory 'regular/sub'

(regular is a plain file.) Both fail and leave the paths alone; only the messages differ. The single-source install -t X f case is handled separately in #14997.

[xtqqczze]

@abendrothj In general, please check against the latest GNU version, preferably by building from source, or at least GNU 9.12 binaries.

[abendrothj]

Rechecked against GNU 9.12 built from the release tarball: all five messages are identical to the 9.1 output in the table, and uutils main (2b85f6f12) still differs in the same way. I've updated the version in the description.