Add optional func to set the span's error tag flag#4

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jmdalmeida wants to merge 3 commits intoopentracing-contrib:masterfrom

jmdalmeida:master

Commits on Dec 6, 2018

Add option func to allow setting the error tag
João Almeida
committed

Commits on Dec 7, 2018

Pass gin's context on error func
João Almeida
committed
Call error func only when explicitly set
João Almeida
committed