Nonexistent memcached command should result in ERROR response, not SERVER_ERROR

From Michal:

"according to the memcached text protocol:
https://github.com/memcached/memcached/blob/master/doc/protocol.txt

sending of an unknown command should result in response "ERROR\r\n"
instead we are sending SERVER_ERROR.

echo -e "blah\r" | nc localhost 11211
SERVER_ERROR org.infinispan.server.core.UnknownOperationException: Unknown operation: blah"