Update docs and error handling

[dbrattli]

• Put response readers in module to avoid collision with Giraffe
• Retry is now a proper HTTP handler. Thus you need to use it before the handler you want to retry.
• Renamed user supplied error to ResponseError.
• Some cleanup.

[codecov[bot]]

Codecov Report

Merging #22 into master will increase coverage by 0.84%. The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   61.53%   62.38%   +0.84%     
==========================================
  Files           9        9              
  Lines         221      218       -3     
  Branches       21       19       -2     
==========================================
  Hits          136      136              
+ Misses         75       72       -3     
  Partials       10       10

Impacted Files	Coverage Δ
src/Encode.fs	32.25% <ø> (ø)	:arrow_up:
src/Error.fs	0% <ø> (ø)	:arrow_up:
src/Fetch.fs	30.18% <0%> (+0.55%)	:arrow_up:
src/Handler.fs	83.63% <100%> (+2.93%)	:arrow_up:
src/Retry.fs	80% <100%> (ø)	:arrow_up: