Most of the time, the return value of this method should be checked,
because behavior should depend on whether or not an exception is
thrown. However, if it is called immediately after a throw it doesn't
need to be checked because it will always return true. bz said there
is no public API that lets you assume there is an exception because it
would be "too easy to misuse".
MozReview-Commit-ID: CqyicBbcNjW