Rethrow exception in Set-AzureNetworkSecurityGroupToSubnet

Right now in Set-AzureNetworkSecurityGroupToSubnet exceptions are being caught in case we can automatically handle them (when a user is trying to change the NSG that applies to a subnet already having an NSG).

However, if the exception is not the one we can handle automatically, we should rethrow it.

All tests are passing now:

![image](https://cloud.githubusercontent.com/assets/637952/5309502/28723eb8-7bda-11e4-9c8f-4f6f2149b02f.png)