What we need to be able to set in myisamchk is
actually a collation, not a character set. This
fix just changes to display the proper error message. 


include/mysys_err.h:
  New mysys error message "unknown collation".
mysys/charset.c:
  Display more proper error when a collation is not found.
mysys/errors.c:
  New "unknown collation" error.