Refactor: Push schema index/constraint conflict messages down the stack to align with SPI.

Proper messages for these exceptions can only be created within the context of the operation
and below the SPI level so that such users (Cypher) enjoy increased friendliness.
NO!!!!!!!

There is great value to be had from having an actual object with semantic value available in the exception object.

Duplicating the message field in a subclass is just bad though.