While this message may be more helpful, it will break some cases, such
as when there are multiple ciphers that could be used and we can just
skip CAST5 and move on. Return NULL when CAST5 unsupported, just like
for other unsupported ciphers.
I'm going to make this one more generic than one library, and have it
support ciphers based on what libraries are available, so this more
generic name is appropriate.