blob: Let readFromWriteTo return the original err

[petr-tichy]

Allows the caller to detect the error type instead of returning a
formatted string

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[codecov]

Codecov Report

Merging #2735 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2735      +/-   ##
==========================================
+ Coverage   68.34%   68.35%   +<.01%     
==========================================
  Files         115      115              
  Lines       13274    13274              
==========================================
+ Hits         9072     9073       +1     
+ Misses       3542     3539       -3     
- Partials      660      662       +2

Impacted Files	Coverage Δ
blob/blob.go	89.85% <0%> (ø)	⬆️
runtimevar/filevar/filevar.go	85.32% <0%> (-1.84%)	⬇️
pubsub/pubsub.go	91.87% <0%> (-1.57%)	⬇️
server/health/sqlhealth/sqlhealth.go	80.95% <0%> (+19.04%)	⬆️

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[petr-tichy]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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