Add check if Pester itself fails

[SteveL-MSFT]

PR Summary

• One of the unit tests wasn't correctly attributed as a test so it produced a warning, but should have failed the build. Fix is to use RUSTFLAGS env var to turn warnings into errors even when building unit tests
• Pester itself can fail and the build script just treats this as success since Pester didn't report an error, wrap in try...catch and throw if Pester has an issue