Add HandlerError::cause and HandlerError::into_cause

[msrd0]

Fixes #574

[codecov]

Codecov Report

Merging #575 (e982e78) into master (ecc604d) will increase coverage by 0.38%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #575      +/-   ##
==========================================
+ Coverage   85.26%   85.65%   +0.38%     
==========================================
  Files         111      111              
  Lines        5768     5722      -46     
==========================================
- Hits         4918     4901      -17     
+ Misses        850      821      -29     

Impacted Files	Coverage Δ
gotham/src/handler/error.rs	66.66% <0.00%> (+1.36%)	⬆️
gotham/src/handler/mod.rs	82.60% <ø> (+3.44%)	⬆️
gotham/src/state/mod.rs	93.44% <ø> (+5.75%)	⬆️
gotham_derive/src/lib.rs	0.00% <ø> (ø)
middleware/template/src/lib.rs	0.00% <ø> (ø)
gotham/src/helpers/http/response.rs	61.90% <0.00%> (-38.10%)	⬇️
examples/hello_world_until/src/main.rs	84.90% <0.00%> (-11.33%)	⬇️
gotham/src/pipeline/single.rs	88.88% <0.00%> (-11.12%)	⬇️
gotham/src/lib.rs	80.76% <0.00%> (-7.24%)	⬇️
gotham/src/test/async_test.rs	67.74% <0.00%> (-5.95%)	⬇️
... and 34 more

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