You are not expected to check the proofs. You are expected to write an informative text which will reflect the contents of the paper. So that the reader can get an idea what did the author prove (or claims to prove), and decide whether s/he wants to read the paper. Of course, if you find a mistake you mention this. Or if you know that the result is not new. But you are not supposed to check the proof: it is the job of the referee.