Changed my_error() to print error messages, which come from
arbitrary registered ranges of error messages. Messages can
be unregistered (and should be at end of the program).
Added registration of handler error messages.
Added a new mi_print_error() macro and a new 
mi_report_error() function, which supply error
messages with a table name.
Added calls to mi_print_error() or mi_report_error()
at all places in MyISAM, where table corruption is detected.