Expose connection info in errors

[sagebind]

Add local_addr and remote_addr to Error which expose the local and remote network addresses of the last used connection for a request before the error occurred, if known.

Fixes #336.

[codecov]

Codecov Report

Merging #337 (897b25d) into master (8ba69f4) will increase coverage by 0.25%.
The diff coverage is 90.56%.

@@            Coverage Diff             @@
##           master     #337      +/-   ##
==========================================
+ Coverage   74.80%   75.05%   +0.25%     
==========================================
  Files          58       58              
  Lines        3131     3179      +48     
==========================================
+ Hits         2342     2386      +44     
- Misses        789      793       +4     

Impacted Files	Coverage Δ
src/response.rs	60.52% <50.00%> (ø)
src/redirect.rs	70.58% <75.00%> (-0.50%)	⬇️
src/error.rs	50.85% <86.95%> (+6.12%)	⬆️
src/handler.rs	73.39% <100.00%> (+0.94%)	⬆️
tests/net.rs	92.42% <100.00%> (+1.51%)	⬆️
tests/redirects.rs	100.00% <100.00%> (ø)
src/client.rs	72.92% <0.00%> (-0.17%)	⬇️

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