Issue #6294 describes an edge case where the browser crash if the same
module is loaded three times in a document, but all attempts fail.
Failure scenario:
1. Module load 1 set the state to "Fetching"
2. Module load 2 registers a callback to `on_complete` since the
current state is "Fetching"
3. Module load 1 finish with a failure, invoking the callback for load
number 2
4. Module load 3 cause a crash. The state is neither "Fetching" or
"ModuleScript", so we'll reset the state to "Fetching". This invokes
the callback for module load 2 again, now with an unexpected state
which will cause an assert violation.
Proposed fix is to remove the condition that invokes `on_complete`
immediately for successfully loaded modules only, the callback should
be invoked regardless of whether the fetch succeeded or failed.
This reveals a separate bug in HTMLScriptElement, where
`mark_as_ready()` can be invoked before
`m_steps_to_run_when_the_result_is_ready` is assigned.
This appears to be a spec bug, reported as
https://github.com/whatwg/html/issues/12073 and addressed by delaying
the callback by a task, similar to the issue was resolved for inline
scripts.
