add more information to debug transcoding fail-case

[oscar-davids]

A follow up to #276.

we won't have any info on the output codec requested (and, potentially, other capabilities, detection, verification ) in target_profiles of JSON files. To know what exactly failed, we should store the complete JSON that is returned via the stream-auth-webhook instead of just the target profiles.

ref link: https://github.com/livepeer/go-livepeer/blob/7af7b79fc0c859f67f0a9318ba5549d7da4b16f6/server/mediaserver.go#L105