use CL_INVALID_OPERATION as the error code for unsupported features (#418)

* use CL_INVALID_OPERATION as the error code for unsupported features

The one case where a different error code may be returned is for
clGetPipeInfo, since ICD implementations cannot create a valid
pipe memory object to get the dispatch table from, and hence will
return CL_INVALID_MEM_OBJECT instead.