Convert error code (int) to string (const char *):
* DMMP_OK -- "OK"
* DMMP_ERR_BUG -- "BUG of libdmmp library"
* DMMP_ERR_NO_MEMORY -- "Out of memory"
* DMMP_ERR_IPC_TIMEOUT -- "Timeout when communicate with multipathd, try to set bigger timeout value via dmmp_context_timeout_set ()"
* DMMP_ERR_IPC_ERROR -- "Error when communicate with multipathd daemon"
* DMMP_ERR_NO_DAEMON -- "The multipathd daemon not started"
* DMMP_ERR_INCOMPATIBLE -- "The multipathd daemon version is not compatible with current library"
* Other invalid error number -- "Invalid argument"