Handle parse errors

[jan4843]

Requests where the body is not valid JSON would cause MultiJson::ParseError to be raised but not translated to a RequestHandler error. Handling RequestHandler::ExternalBaseError should arguably include parsing errors caused by invalid user input.

[codecov-commenter]

Codecov Report

Merging #47 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   98.19%   98.20%           
=======================================
  Files          41       41           
  Lines         887      889    +2     
=======================================
+ Hits          871      873    +2     
  Misses         16       16           

Impacted Files	Coverage Δ
lib/request_handler/body_parser.rb	100.00% <100.00%> (ø)
lib/request_handler/error.rb	100.00% <100.00%> (ø)

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