Audit debug-only asserts and ensure calls to exit the process are distinguishable

There are some places where a debug-build-only ASSERT should be checked even in release build. An example is that during some parts of attach/detach, DR should really verify that threads are in a safe spot lest detach silently fail and cause a crash later on during application execution. In addition, there are a few places in the code I've seen which exit the process or thread without an associated message that I think should annotated. Derek suggested changing the declaration of OS-level calls to terminate the process to include a text reason of why the process is being terminated.