Msbuild: Log our own error if a task returned false but didn't log any errors

[KirillOsenkov]

If a task violates the implicit expectation that it should return false iff it logged any errors MSBuild fails the build, but doesn't indicate why.

We should consider at least adding a warning, but ideally failing the build (perhaps in strict mode?)

[jeffkl]

@KirillOsenkov We had discussed overriding a task result if it logged errors but decided it would have too large of an impact. I haven't actually seen a task that logs an error but doesn't return false. Have you hit this a lot?

[KirillOsenkov]

Just once, but it hurt enough ;) The symptoms is you get all green/gray output (no red), and a Build failed in the end. Good luck.

Structured Log Viewer just showed a Build failed at the end with no other errors. Had to painstakingly read everything until I stumbled at a message from the task at fault that gave me the hint.

We definitely need at least a warning here.

[KirillOsenkov]

Oh, and its the other case: it returns false, but doesn't log an error.

[rainersigwald]

We've definitely talked about this before but I couldn't find a bug. It does seem entirely reasonable to inject an "error MSBXXXX: Task X returned failure but did not log an error" while still in the logging context of the task. Not sure how easy that would be.

[KirillOsenkov]

Note that this wouldn't be a breaking change because their build fails anyway, it's just the log doesn't have any errors displayed.

[atanasa]

Does the log contain the value of what the task returned? If not, maybe that could be something to consider.

[rainersigwald]

Let's just do this in the first preview of 16.5. YOLO and this keeps biting people. Any objections?

[KirillOsenkov]

:heart:

[livarcocc]

Sounds good to me.

[rainersigwald]

We pulled this out of 16.6 because of #5203. Reactivating--we'll aim for 16.7.