go vet warnings

[chrisaxiom]

Hello!

Thank you for the great library - we use it daily. Recently we ran into a small issue.

This is a go vet bug in my opinion, but until that is addressed, people running go vet on their code (including /vendor folder) will run into the following warning when using the Error handler:

arg (*Context).Error in Error call is a function value, not a function call

You can also see this by running go vet in the root of this project. I think go vet is incorrectly picking up keywords here (error, panic, etc) and incorrectly reporting that warning, as renaming Error to Panic also fails, where OnError passes (for example).