Tell why a request is bad for a 400 Bad Request response

[trustin]

Motivation:

It is sometimes hard for a client to know why a request is bad.

Modifications:

• Added short error messages to 400 Bad Request responses
• Used ProtocolViolationException instead of IllegalArgumentException for bad requests.
• Removed unnecessary validation of HTTP method in HttpServerHandler.
• Cleaned up response generation in HttpServerHandler.

Result:

• User friendliness.

[trustin]

/cc @jwills

[codecov]

Codecov Report

Merging #1575 into master will increase coverage by <.01%.
The diff coverage is 67.02%.

@@             Coverage Diff              @@
##             master    #1575      +/-   ##
============================================
+ Coverage     72.77%   72.77%   +<.01%     
- Complexity     7495     7499       +4     
============================================
  Files           695      695              
  Lines         30171    30188      +17     
  Branches       3683     3687       +4     
============================================
+ Hits          21956    21970      +14     
- Misses         6310     6317       +7     
+ Partials       1905     1901       -4

Impacted Files	Coverage Δ	Complexity Δ
...spring/web/reactive/ArmeriaHttpHandlerAdapter.java	75% <0%> (-3.95%)	5 <0> (ø)
...m/linecorp/armeria/server/Http2RequestDecoder.java	59.68% <50%> (+2.54%)	24 <1> (+1)	⬆️
.../linecorp/armeria/server/tomcat/TomcatService.java	63.82% <62.5%> (+0.09%)	24 <0> (ø)	⬇️
...m/linecorp/armeria/server/Http1RequestDecoder.java	76.76% <66.66%> (+1.76%)	27 <0> (+1)	⬆️
...com/linecorp/armeria/server/HttpServerHandler.java	78.7% <79.54%> (+1.52%)	79 <11> (+5)	⬆️
...rp/armeria/server/tomcat/ManagedTomcatService.java	50.84% <0%> (-11.87%)	6% <0%> (-1%)
...armeria/server/tomcat/Tomcat90ProtocolHandler.java	48.14% <0%> (-7.41%)	11% <0%> (-2%)
...lient/circuitbreaker/CircuitBreakerHttpClient.java	69.69% <0%> (-3.04%)	10% <0%> (-1%)
.../linecorp/armeria/client/Http1ResponseDecoder.java	58.18% <0%> (-1.82%)	22% <0%> (-1%)
...om/linecorp/armeria/client/HttpSessionHandler.java	57.75% <0%> (-1.73%)	27% <0%> (-1%)
... and 6 more

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[minwoox]

LGTM 👍

[trustin]

Thanks for reviewing.