For most system errors MySQL will, in addition to a internal text message, also print the system error code in one of the following styles: message ... (errno: #) or message ... (Errcode: #).
You can find out what the error code means by either examining the documentation for your system or use the perror utility.
perror prints a description for a system error code, or an MyISAM/ISAM storage engine (table handler) error code.
perror is invoked like this:
shell> perror [OPTIONS] [ERRORCODE [ERRORCODE...]] Example: shell> perror 13 64 Error code 13: Permission denied Error code 64: Machine is not on the network
Note that the error messages are mostly system dependent!