Update port conflict checker to distinguish between conflicts and exact matches.

This is a fix for #1359837 (https://bugs.launchpad.net/juju-core/+bug/1359837).

Opening a port that is already open should not result in an error.
I second this, it sounds like the error should be the same for both, "port range conflict".

1. people shouldn't be sniffing the error value to see if they overlapped partially or fully with an existing range
2. this probably shouldn't be a constant error, I would prefer that an error be constructed at the time the conflict is detected and as much detail as humanly possible returned as I believe this is going to be a support boat anchor.