go1.7 vet flags "possible formatting directive" when Error is used

[prashantv]

When the assert.Error method is used with formatting arguments, go vet complains as of 1.7:

flag_test.go:61: possible formatting directive in Error call

For more information and a repro, see:
uber-go/zap#135

The specific check that is run is here:
https://golang.org/src/cmd/vet/print.go#L587

I'm not sure if there's a good fix for this, but I wanted to flag this, as I typically:

• recommend adding context to the assert.Error calls
• suggest usage of assert.Error instead of assert.NotNil

However, this "correct" usage of the library leading to a vet failure may mean users will either avoid one or the other.

[ernesto-jimenez]

@prashantv this will require creating methods like Errorf and Equalf as counterparts of Error and Equal with formatted messages.

Let me look into it :)

[ernesto-jimenez]

@prashantv what does #356 look like?

[prashantv]

Thanks @ernesto-jimenez, left some comments but overall looks good!