Add some logging to 3pid invite sig verification (#5015)

I had to add quite a lot of logging to diagnose a problem with 3pid
invites - we only logged the one failure which isn't all that
informative.

NB. I'm not convinced the logic of this loop is right: I think it
should just accept a single valid signature from a trusted source
rather than fail if *any* signature is invalid. Also it should
probably not skip the rest of middle loop if a check fails? However,
I'm deliberately not changing the logic here.