WiP: call `failureBlock` during `setExternalUserId` if one or more channels return success of false

[nan-li]

• we were always calling the successBlock after making the request to set external user ID
• which means we call the successBlock even when the backend doesn't set it and return the error of invalid auth hath
• now we check if the result has any channels that returned "success" of false and call the failureBlock
• (to remove later) I made some comments in the code about what to do for certain things

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