Return errors instead of pushing down.

[dbrattli]

• Short circuit error handling by returning error directly instead of passing down the line.
• Start adding unit-tests for handlers.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@518af13). Click here to learn what that means. The diff coverage is 9.09%.

@@            Coverage Diff            @@
##             master       #9   +/-   ##
=========================================
  Coverage          ?   14.69%           
=========================================
  Files             ?        8           
  Lines             ?      279           
  Branches          ?       29           
=========================================
  Hits              ?       41           
  Misses            ?      238           
  Partials          ?        0

Impacted Files	Coverage Δ
src/Error.fs	19.44% <ø> (ø)
src/Handler.fs	13.95% <0%> (ø)
src/Builder.fs	0% <0%> (ø)
src/Encode.fs	0% <0%> (ø)
src/Context.fs	68.29% <50%> (ø)