Support warnings as errors

[mxcl]

However, this should only be applied to build targets, not tests

[mxcl]

Easiest way to do this is two invocations of xcodebuild, one with this enabled for build, then without for test

[mxcl]

This is added, however we need to do better ideally by editing the project and setting this setting only for the targets of our app.

Because otherwise warnings in Swift package dependencies cause build failure.

This would also mean we can stop doing two separate instantiations of xcodebuild if this flag is enabled.