Fail when trace.length > 2

[fregante]

Additional safety check following:

• Unwanted forwarding when targeting allFrames #219

This is not the right solution to the issue since it will trigger an error, but it should avoid loops in the future.

[twschiller]

This is not the right solution to the issue since it will trigger an error

I think this check is a good fail-safe

Is another guardrail that sender should ignore/drop/not forward any messages where it would be forwarding it to the same target as itself?

[fregante]

I can leave a comment for that in the future, but if I limit forwarding to the background page then neither one of these situations should happen