SI-7319 Clear error buffer during Typer reset.

And warn under -Xdev, because we shouldn't get here.

In the enclosed test, the error buffer of `startContext`
contained a reported but unflushed type error.

I believe the culprit is `typerReportAnyContextErrors`,
which will be addressed in the next commit.
SI-7319 Flush error buffer in typerReportAnyContextErrors.

After this change, we no longer rely on the backstop in resetContexts
introduced in the previous commit.