cleanup errors

[jmank88]

What does this do / why do we need it?

Just a bit of error cleanup.

• Leverage implicit ProjectIdentifier.String()
• Swap in errors.Wrap... when tacking the error on as the last argument to Errorf

[darkowlzz]

Can we also replace all the fmt.Errorf() with errors.Wrapf()?
Like in
https://github.com/golang/dep/blob/master/cmd/dep/status.go#L380
https://github.com/golang/dep/blob/master/internal/gps/source_manager.go#L176
https://github.com/golang/dep/blob/master/internal/gps/source_manager.go#L223

[darkowlzz]

LGTM!

[carolynvs]

@darkowlzz Shoot, sorry I just realized that you actually asked for more changes... 😊