protoc-gen-swagger: add default error response

[johanbrandhorst]

Adds a "default" error entry to all responses
in the swagger definitions.

Fixes #1122

I can't think of a reason why this wouldn't be backwards compatible.

[codecov-io]

Codecov Report

Merging #1127 into master will increase coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1127      +/-   ##
==========================================
+ Coverage   53.68%   53.87%   +0.18%     
==========================================
  Files          42       42              
  Lines        4187     4197      +10     
==========================================
+ Hits         2248     2261      +13     
+ Misses       1692     1689       -3     
  Partials      247      247

Impacted Files	Coverage Δ
runtime/errors.go	49.31% <100%> (+1.94%)	⬆️
protoc-gen-swagger/genswagger/template.go	56.88% <100%> (+0.51%)	⬆️

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[achew22]

This may have some knock on implications for Bazel, but it's totally fine. I'll deal with it once this is merged.

[seanlaff]

@johanbrandhorst Thanks for this!