Add support to treat class/assembly warnings as errors

[NGloreous]

Assembly/Class cleanup errors mostly go unnoticed today (they get logged as a warning which for most automated CI systems means nobody sees the warning since it doesn't fail the test run). Even if the warning was surfaced, any output from these cleanups are discarded so it's harder to debug the issue.

There are two parts to this change:

• Add a setting to treat assembly/class cleanup warnings as errors to cause the test run to fail
• Log stdout & stderr captured during cleanup to help in debugging.