$ SHELL=/doesnotexist src/emacs -Q -batch -l htmlfontify ; echo $?
Searching for program: No such file or directory, /doesnotexist
255

This is of course a rather rare edge case, but I think that
unavailability of a shell shouldn't prevent htmlfontify from loading.
The shell is only used to initialize hfy-etags-cmd, this variable could
simply be initialized to nil if no shell is found, similar to when no
etags binary is found.