remove conditional error when disabling, treating warnings as errors

[DaveSkender]

Fixes #266, partially addresses #235

As someone who a) wants to disable mutation of source code on my build server, because it's a volatile thing that I don't want, and also b) wants to be conservative and treat my [unintentional] warnings as errors, I cannot do that because this always fails:

dotnet build --property:RtDisable=true --property:TreatWarningsAsErrors=true

The recommendation here is to simply decouple these properties and only Message when something is disabled as an intentional act.

Also consider:

• Alternate Warning type with a unique error code for suppression, see RT and TreatWarningsAsErrors #235. I've opted for Message type in my recommendation.