Improve logging when event-signature checking fails

[richvdh]

There are basically two changes here:

• The change in keyring.py is a refactor of verify_json_objects_for_server, the main purpose of which is that when there are no signatures at all from the relevant server, we shortcut _handle_key_deferred and instead just return a failure which tells us what happened.

• The changes in federation_base catch the failures returned by verify_json_objects_for_server and turn them into new exceptions which tell you which event caused the problem.

[codecov]

Codecov Report

Merging #5100 into develop will decrease coverage by 0.01%.
The diff coverage is 50%.

@@             Coverage Diff             @@
##           develop    #5100      +/-   ##
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- Coverage    61.67%   61.65%   -0.02%     
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  Files          334      334              
  Lines        34512    34521       +9     
  Branches      5680     5680              
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  Hits         21284    21284              
- Misses       11699    11708       +9     
  Partials      1529     1529