Fail messages for unknown partitions immediately (issue #39)

Two error paths:
 - produce() will fail with errno set to ESRCH
 - dr_cb() will provide err RD_KAFKA_RESP_ERR__UNKNOWN_PARTITTION
Changed msgtimeout test to unknown partition test (issue #39)

The way msgtimeout test was implemented will not work any more with
the fix for issue #39
Clarified comments and function argument names.