C#: Add option to treat warnings as errors

[Repiteo]

With CI now able to detect warnings/errors from MSBuild operations, the ability to outright fail a CI if such warnings/errors crop up has become much more important. This integrates exactly that, by adding a new argument --werror to the mono build_assemblies.py, which functions like werror=yes on SCons. This argument is now passed in the Linux mono CI, and the two existing warnings have been addressed so builds will succeed.

[raulsntos]

Looks good to me, thanks! I tested it locally and works as expected, I'll be passing --werror in my build scripts so I don't miss new warnings when testing PRs from now on.

[akien-mga]

Thanks!