fix: return error when sync context ends

[rainest]

If a sync context ends, return its Context.Err() in the list of sync errors. This informs downstream clients that the sync did not process all entities, even if there were no events in flight (which generate their own errors, as seen below) at the time the context ended. On the user end, this will look like:

11:52:27-0800 esenin $ /tmp/bdeck sync -s /tmp/foo.yaml
creating consumer foo
...
creating upstream foo.8080.svc
^Creceived interrupt , terminating...
Summary:
  Created: 17
  Updated: 0
  Deleted: 0
Error: 5 errors occurred:
	sync context ended, some entities were not synced: context canceled
	while processing event: {Create} upstream bar.8001.svc failed: making HTTP request: Put "http://localhost:8001/upstreams/3acc5ce0-6a3b-40d2-a63c-6ab73d1db221": context canceled
        ...

Fixes #528

[codecov-commenter]

Codecov Report

Merging #529 (837046a) into main (a01c962) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #529      +/-   ##
==========================================
- Coverage   50.47%   50.46%   -0.01%     
==========================================
  Files          72       72              
  Lines        7870     7871       +1     
==========================================
  Hits         3972     3972              
- Misses       3549     3550       +1     
  Partials      349      349              

Impacted Files	Coverage Δ
diff/diff.go	0.00% <0.00%> (ø)

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[hbagdi]

I'm not certain if this will indeed solve the issue or not but is minor enough to give it a try.