exit with status 1 if grpcping -health isn't SERVING.

[ldemailly]

fixes #588, #456, #595 (invalid utf8 when random payload is used)

testing through newly added _down extra service:

$ go run . grpcping -health -healthservice ping_down -n 10 localhost
RTT histogram usec : count 10 avg 1415.4499 +/- 3828 min 124.292 max 12899.791000000001 sum 14154.499
# range, mid point, percentile, count
>= 124.292 <= 140 , 132.146 , 70.00, 7
> 140 <= 160 , 150 , 80.00, 1
> 180 <= 200 , 190 , 90.00, 1
> 10000 <= 12899.8 , 11449.9 , 100.00, 1
# target 50% 134.764
Health NOT_SERVING : 10
11:18:17 E fortio_main.go:526> Unexpected SERVING count 0 vs 10
exit status 1

[codeclimate]

Code Climate has analyzed commit e64d34b and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #589 (e64d34b) into master (56fa5ab) will decrease coverage by 0.0%.
The diff coverage is 95.2%.

@@           Coverage Diff            @@
##           master    #589     +/-   ##
========================================
- Coverage    87.5%   87.5%   -0.0%     
========================================
  Files          26      26             
  Lines        3663    3674     +11     
========================================
+ Hits         3206    3215      +9     
- Misses        305     306      +1     
- Partials      152     153      +1     

Impacted Files	Coverage Δ
fhttp/http_client.go	83.5% <95.0%> (-0.1%)	⬇️
fgrpc/pingsrv.go	89.0% <100.0%> (+0.1%)	⬆️

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