Because HideNonSensicalMessages looks at hasErrors, we need to narrow
the faking to just the isNonSensical call

We can switch to a more straightforward hasErrors check in isHidden, but
then we need to bump the errorCount before calling doReport, as doReport
will then, necessarily, force the message.

For reference, the way I test this manually is by:
  1. In ignoredConvertibleImplicits, changing back to `viewExists(imp, fail.expectedType)`
  2. In adapt1, removing the `dummyTreeOfType.unapply(tree).isEmpty` guard
  3. Compiling tests/neg/i18650.scala

Also, while I'm here, instruct on the presence of
-Yno-enrich-error-messages, like we do for -Yno-decode-stacktraces.