When performing a move DN operation in the AD connector, check that the target OU is present first

When the target OU is missing, the error that is currently logged is quite difficult to relate back to the root cause.  If a test of the target OU were made before the move and a specific error logged in this case then it would go a long way to improving the usability of the product.

See https://voice.unifysolutions.net/communities/6/topics/3850-baseline-sync-reports-received-error-code-other-an-unknown-error-occurred for an example of the error that currently occurs.