Improved unimplemented API call error message.
Improved unimplemented API call error message.

mpflanzer/Oclgrind,surround-io/oclgrind,mpflanzer/Oclgrind,surround-io/oclgrind,bitfusionio/Oclgrind,mpflanzer/Oclgrind,surround-io/oclgrind,surround-io/oclgrind,bitfusionio/Oclgrind,mpflanzer/Oclgrind,bitfusionio/Oclgrind,bitfusionio/Oclgrind