Return a special error when watcher channel is closed

[jimmykarily]

so consumers can act upon the error as they desire (reconnect?)

[jimmykarily]

A consumer can use a block like this to filter for "watcher channel closed" error:

if err, ok := err.(*extension.WatcherChannelClosedError); ok {
// Do things
}

[codecov]

Codecov Report

Merging #19 into master will increase coverage by 2.96%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
+ Coverage   71.09%   74.05%   +2.96%     
==========================================
  Files           3        3              
  Lines         422      424       +2     
==========================================
+ Hits          300      314      +14     
+ Misses         98       82      -16     
- Partials       24       28       +4

Impacted Files	Coverage Δ
manager.go	69.65% <100%> (+6.33%)	⬆️

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