Feature: Implement gRPC error assertion

[cyantarek]

No description provided.

[dnephin]

https://godoc.org/gotest.tools/assert#ErrorType accepts a func(error) bool which can be used with a small local function in your test package for checking gRPC errors:

func grpcErrorCode(code codes.Code) func(error) bool {
    return func(err error) bool {
        return status.Code(err) == code
    }
}

...

assert.ErrorType(t, err, grpcErrorCode(codes.NotFound))

This is probably a good example for the documentation.