ai/live: Handle more special cases in trickle.

[j0sh]

ai/live: More error handling for gotrickle clients.

• Handles additional stopping conditions in publisher such as 404
• Handles "stream exists but segment doesn't" condition in subscriber

These lead to 1) more timely shutdowns when necessary, and 2) faster recovery if a subscription falls behind

ai/live: Close a segment if it will be dropped.

This is being polite to notify any subscribers that might be
waiting for the segment that the segment will not be around.

Not required if the segment is written normally, and subscribers
will still work fine without; it would just take longer to happen
when the segment drops out of the window of active segments.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 98 lines in your changes missing coverage. Please review.

Project coverage is 33.75274%. Comparing base (2ff0054) to head (09a77a2).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
trickle/trickle_server.go	0.00000%	38 Missing ⚠️
trickle/trickle_subscriber.go	0.00000%	27 Missing ⚠️
trickle/trickle_publisher.go	0.00000%	20 Missing ⚠️
server/ai_live_video.go	0.00000%	13 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3332         +/-   ##
===================================================
- Coverage   33.83786%   33.75274%   -0.08512%     
===================================================
  Files            141         141                 
  Lines          37263       37354         +91     
===================================================
- Hits           12609       12608          -1     
- Misses         23934       24026         +92     
  Partials         720         720                 

Files with missing lines	Coverage Δ
server/ai_live_video.go	0.00000% <0.00000%> (ø)
trickle/trickle_publisher.go	0.00000% <0.00000%> (ø)
trickle/trickle_subscriber.go	0.00000% <0.00000%> (ø)
trickle/trickle_server.go	0.00000% <0.00000%> (ø)

... and 1 file with indirect coverage changes

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Files with missing lines	Coverage Δ
server/ai_live_video.go	0.00000% <0.00000%> (ø)
trickle/trickle_publisher.go	0.00000% <0.00000%> (ø)
trickle/trickle_subscriber.go	0.00000% <0.00000%> (ø)
trickle/trickle_server.go	0.00000% <0.00000%> (ø)

... and 1 file with indirect coverage changes