Proper handling when portal is being rejected

[JeDaYoshi]

Related to crybapp/portals#8. Portal should handle properly if it is being rejected from portals, instead of infinitely trying to connect. Probably @cryb/portals would work somewhat better if we did give some hints as of why it is being rejected instead of just returning nothing.

[darnfish]

Was thinking of having better documentation when this type of crash occurs in development, I'll see what I can do 😄