When a query is terminated due to exceeding its maximum execution time
specified using the MAX_EXECUTION_TIME hint, the error produced will differ
depending on the query stage at termination. Most queries will produce
ER_QUERY_TIMEOUT (3024), but if a query is terminated during filesort,
ER_FILSORT_ABORT (1028) will be produced instead.

This makes it unnecessarily harder to trap these errors and respond
correctly.

The fix is to remove the two special error codes ER_FILSORT_ABORT and
ER_FILESORT_TERMINATED. filesort() is now a normal part of the
execution engine, and query termination (timeout or killed) will be
reported in the standard way.

Change-Id: I6afdbf6937d2f260005b336b17f23c8125b275fb