See WT-17582.

As part of the agreement for exposing and using these WT metrics, WT has agreed to add a new error code (along the lines of WT_OVERLOAD_REJECT or similar). Server must now handle this and return some exception.

Also to think about: how different callers with different priorities might want to handle this error code return differently.

Note: this ticket must be merged in conjunction with the WT ticket that changes it to return the new error code.