Add missing error code to RowError

[kolodny]

Fixes #5

There was a missing err.code assignment that was missing in RowError, Family has a similar snippet.

Also added tests around the row.exists() method.

[googlebot]

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[callmehiphop]

@kolodny I went ahead and made a commit for unit tests, if it looks good to you I'll merge this asap.

[kolodny]

Looks good to me!

[codecov-io]

Codecov Report

Merging #11 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #11   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           8      8           
  Lines         791    792    +1     
=====================================
+ Hits          791    792    +1

Impacted Files	Coverage Δ
src/table.js	100% <ø> (ø)	⬆️
src/row.js	100% <100%> (ø)	⬆️

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