Defer errors for all iterators

[flimzy]

Fixes #616

[codecov]

Codecov Report

Patch coverage: 96.87% and project coverage change: +0.33 🎉

Comparison is base (51b82d6) 92.13% compared to head (67fbd80) 92.47%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #619      +/-   ##
==========================================
+ Coverage   92.13%   92.47%   +0.33%     
==========================================
  Files          21       21              
  Lines        1323     1329       +6     
==========================================
+ Hits         1219     1229      +10     
+ Misses         78       76       -2     
+ Partials       26       24       -2     

Impacted Files	Coverage Δ
bulk.go	97.14% <88.88%> (+5.96%)	⬆️
changes.go	100.00% <100.00%> (ø)
iterator.go	97.70% <100.00%> (+0.11%)	⬆️
resultset.go	76.64% <100.00%> (ø)
updates.go	100.00% <100.00%> (ø)

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