If we have a creator parser, then we were a parser-inserted script and should
presumably be able to set a valid insertion point when we run or fire our
load/error events. For the error event case, we do this in
nsScriptElement::ScriptAvailable, so that async error events due to things like
bogus script URLs do not end up with a valid insertion point. For the load
event case, we just do this in ScriptEvaluated directly.
ScriptEvaluated is called while the scriptloader has our script set as the
current parser-inserted script. But for the error event case we need to
maintain that state around the ScriptAvailable call that will fire the event.
