fix: handle parsing & primary error properly

[vigneshshanmugam]

• If we respond with a empty message with Content-Type: 'text/html', the browser will throw error stating as This page isn’t working losing the original error page.
• Now we respond with 500 status code and Internal Server Error as the response when there is a parse error or when there is no fallback provided for primary fragment.

This is a very old bug and it was introduced when we started supporting fallbacks and I noticed after we started the Opentracing instrumentation, so Thanks to @lmineiro 😄

[codecov]

Codecov Report

Merging #235 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
+ Coverage   99.05%   99.06%   +<.01%     
==========================================
  Files          16       16              
  Lines         638      639       +1     
  Branches      117      117              
==========================================
+ Hits          632      633       +1     
  Misses          6        6

Impacted Files	Coverage Δ
lib/request-handler.js	98.97% <100%> (+0.01%)	⬆️

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[vigneshshanmugam]

👍

[DeTeam]

👍