[html5/webrtc] Fix tryGenerateIceCandidates rejections, put it behind a flag

Fixed two occurrences where the tryGenerateIceCandidates workaround rejected without an error, which borked the callers error handling

Also put it behind a config flag. This workaround used to be important when Kurento didnt infer prflx candidates properly, but that`s no longer the case. With the flag, we can disable the workaround to see if there`s any visible regression and hopefully remove it down the road