NUMBER ALREADY USED BY 5.6

The problem was that the patch for Bug#13004581 added a new error
message to 5.5. This causes it to use an error number already used
in 5.6 by ER_CANNOT_LOAD_FROM_TABLE_V2. Which means that error
message number stability between GA releases is broken.

This patch fixes the problem by removing the error message and
using ER_UNKNOWN_ERROR instead.