fix(panic): Stop panicking on shutdown in the syncer and network init

[teor2345]

Motivation

I saw another panic on shutdown in the syncer while preparing for the Zebra showcase.

Complex Code or Requirements

This is the same kind of bug as the last one: a panic on JoinError::Cancelled, which should just be a log.

Solution

Log shutdown errors rather than panicking in the syncer and when initialising zebra-network.

Review

This should be fixed soon, but it's not a release blocker.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[codecov]

Codecov Report

Merging #7104 (29637b3) into main (a6731d1) will decrease coverage by 0.11%.
The diff coverage is 6.06%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7104      +/-   ##
==========================================
- Coverage   77.41%   77.30%   -0.11%     
==========================================
  Files         310      310              
  Lines       41795    41827      +32     
==========================================
- Hits        32354    32335      -19     
- Misses       9441     9492      +51     

[arya2]

Looks good.