allow Authenticators to throw ServiceException

[tangiel]

This feature request lets Authenticators throw ServiceExcpetions, which are then written in the standard JSON format. This change also refactors exception handling when calling an API method to clean up the logic a bit, and change the logging level so that 5xx-level ServiceExceptions log at the WARNING level, and others at INFO.

[codecov-io]

Codecov Report

Merging #59 into master will increase coverage by 0.07%. The diff coverage is 53.84%.

@@             Coverage Diff              @@
##             master      #59      +/-   ##
============================================
+ Coverage     79.68%   79.75%   +0.07%     
+ Complexity     1622     1619       -3     
============================================
  Files           154      154              
  Lines          5438     5428      -10     
  Branches        700      698       -2     
============================================
- Hits           4333     4329       -4     
+ Misses          832      827       -5     
+ Partials        273      272       -1

Impacted Files	Coverage Δ	Complexity Δ
.../server/spi/request/ServletRequestParamReader.java	84.87% <ø> (ø)	22 <0> (ø)	:x:
...n/java/com/google/api/server/spi/request/Auth.java	77.5% <ø> (ø)	10 <0> (ø)	:x:
...ver/spi/request/RestServletRequestParamReader.java	92.59% <ø> (ø)	21 <0> (ø)	:x:
.../java/com/google/api/server/spi/SystemService.java	83.5% <53.84%> (+1.23%)	37 <6> (+1)	:white_check_mark:
...oogle/api/server/spi/handlers/ExplorerHandler.java	100% <0%> (+23.07%)	3% <0%> (-4%)	:x:

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