[errors] return errors instead of exiting process

[tmc]

This opts to return error values instead of allowing sybil.Error to shut the process down via log.Fatalln.

I think the most likely way this change will introduce a regression is that it will not allow any errors in a set of records to go ignored. I would say it carries moderate risk.

Fixes #74

[codecov-io]

Codecov Report

Merging #79 into master will decrease coverage by 0.66%. The diff coverage is 22.98%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
- Coverage   45.34%   44.67%   -0.67%     
==========================================
  Files          47       48       +1     
  Lines        4241     4407     +166     
==========================================
+ Hits         1923     1969      +46     
- Misses       2130     2211      +81     
- Partials      188      227      +39

Flag	Coverage Δ
#fuzztests	0.58% <0%> (-0.03%)	:arrow_down:
#unittests	52.36% <38.52%> (+0.09%)	:arrow_up:

Impacted Files	Coverage Δ
src/sybil/debug.go	14.28% <ø> (+3.17%)	:arrow_up:
src/cmd/cmd_query.go	0% <0%> (ø)	:arrow_up:
src/sybil/printer.go	0% <0%> (ø)	:arrow_up:
src/sybil/table_trim.go	0% <0%> (ø)	:arrow_up:
src/cmd/cmd_ingest.go	0% <0%> (ø)	:arrow_up:
src/cmd/cmd_index.go	0% <0%> (ø)	:arrow_up:
src/sybil/config.go	55.55% <0%> (+3.83%)	:arrow_up:
src/sybil/errors.go	0% <0%> (ø)
src/cmd/cmd_trim.go	0% <0%> (ø)	:arrow_up:
src/sybil/node_aggregator.go	0% <0%> (ø)	:arrow_up:
... and 18 more

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[tmc]

ok changed integration target to newly cut grpc branch