protoc-gen-swagger: add flag to disable default errors

[johanbrandhorst]

The new flag disable_default_errors can be used to disable
the generation of the default error types. This is suitable
for users that define their own error formatting.

[johanbrandhorst]

Tested this by manually generating all the swagger files with this enabled and it produces the expected output.

[codecov-io]

Codecov Report

Merging #1141 into master will decrease coverage by 0.05%.
The diff coverage is 72.22%.

@@            Coverage Diff             @@
##           master    #1141      +/-   ##
==========================================
- Coverage   53.79%   53.73%   -0.06%     
==========================================
  Files          42       42              
  Lines        4207     4213       +6     
==========================================
+ Hits         2263     2264       +1     
- Misses       1696     1701       +5     
  Partials      248      248

Impacted Files	Coverage Δ
protoc-gen-swagger/main.go	26.6% <0%> (-0.25%)	⬇️
protoc-gen-grpc-gateway/descriptor/registry.go	56.77% <0%> (-0.98%)	⬇️
protoc-gen-swagger/genswagger/template.go	56.76% <100%> (+0.03%)	⬆️

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[jgiles]

LGTM