Ignore error thrown by dispatchDOMEvent

Ignore error thrown by dispatchDOMEvent

Triggered e.g. by `f`-ing on an extension card's description/title. The
click still registers, so it's just noise in the log. This bug is
present in all supported versions (at least 68..73), but unlikely to be
fixed, as it is "only" a testing function (and we're not supposed to use
it).
NS_ERROR_UNEXPECTED: Component returned failure code: 0x8000ffff (NS_ERROR_UNEXPECTED) [nsIDOMWindowUtils.dispatchDOMEventViaPresShellForTesting]