Capture flush errors

[jipperinbham]

while completing the integration tests migration from travisci to semaphoreci, it was discovered that bulk flushes occurring in the go routine were erroring and not bubbling up. the following change attempts to both log and stop transporter when a bulk flush error occurs outside of the Write func.

• CHANGELOG.md updated

[codecov]

Codecov Report

Merging #399 into master will decrease coverage by 0.61%.
The diff coverage is 18.51%.

@@           Coverage Diff            @@
##           master   #399      +/-   ##
========================================
- Coverage   84.62%    84%   -0.62%     
========================================
  Files          59     59              
  Lines        4084   4102      +18     
========================================
- Hits         3456   3446      -10     
- Misses        467    486      +19     
- Partials      161    170       +9

Impacted Files	Coverage Δ
adaptor/rethinkdb/writer.go	75.47% <10%> (-7.7%)	⬇️
adaptor/mongodb/bulk.go	74.22% <12.5%> (-6.21%)	⬇️
adaptor/elasticsearch/clients/v2/writer.go	80.72% <25%> (-2.83%)	⬇️
adaptor/elasticsearch/clients/v5/writer.go	80.72% <40%> (-2.83%)	⬇️
adaptor/postgres/tailer.go	78.21% <0%> (-2.98%)	⬇️

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