Correct loading logic for Exchange Powershell snapin

[markpizz]

It seemed that the intent of the original logic was to allow the Exchange
snapin to be loaded externally or previously and if it hasn't yet been
loaded, then do it automatically.

The original code didn't quite achieve this, there were two errors:

1. The test of the existence somewhere on the machine for the Exchange snapin should be a -Registered check, but it was coded as without -Registered and thus only succeeded if the snapin was previously loaded.
2. The second check should have tested whether it had been loaded, but it merely tested to see if it existed (-Registered) and thus always attempted to load the snapin which resulted in messages about it already being loaded.

[rmbolger]

Thanks again, @markpizz. This function was submitted by a user and I haven't worked with snap-ins all that much which is why I probably missed it during review. Your explanation and fix seems logical.