Display internal hint failures differently than errors/warnings

[antross]

Today if an internal piece of a hint fails it emits an error/warning in the results. This can be a bit confusing as there's not anything necessarily wrong with the content being analyzed.

If we report this differently internally (e.g. using Severity.off for internal failures to indicate a hint failed to run) we can surface this in the UI of the online scanner and browser extension to more clearly communicate that certain hints had an issue executing versus issues with the target page.

[masi]

Like in other tests systems they should be reported as "failed".