In create_myisam_from_heap() mark all errors as fatal except 
HA_ERR_RECORD_FILE_FULL for a HEAP table.

Not doing so could lead to problems, e.g. in a case when a
temporary MyISAM table gets overrun due to its MAX_ROWS limit
while executing INSERT/REPLACE IGNORE ... SELECT. 
The SELECT execution was aborted, but the error was 
converted to a warning due to IGNORE clause, so neither 'ok' 
nor 'error' packet could be sent back to the client. This 
condition led to hanging client when using 5.0 server, or 
assertion failure in 5.1.