What basic principle ensures that the transcribed RNA molecule is carrying the rite genetic message?

Well .... RNA doesn't have the fault-checking that DNA does, so you can't be as sure that it's the right genetic message. This is the reason that RNA-based viruses (HIV, the flu, the common cold, etc) mutate so rapidly as opposed to DNA-based viruses (). The transcription process is basically just base-pairing, is that what you mean?