Make warnings as errors apply only to release builds

[idg10]

We handle warnings as errors to enforce the requirement that we don't check in code that produces warnings. However, this has a negative impact on the developer inner loop. Making small tweaks to code during experimentation becomes much more onerous because you often end up needing to make a bunch of extra changes to keep stylecop and other analyzers happy, for code you may well end up discarding.

To deal with this, we will change to treating warnings as errors only on for release builds. This will continue to prevent code with warnings from being merged in—if developers push code that produces warnings, these will become errors in the CI builds, which will then block the relevant PR from being merged. But it will enable developers to work with warnings during development, fixing them later on as experimentation gives way to writing the final code.