I found this in the doc files:
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 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!
Not very useful, but that's all there was. There is not even a list of error codes.
I'd like to know myself what this "perror" stuff is (sounds like something from C) so I could figure out why I can't select my database.