Throw error when no interfaces are provided

[keddad]

Hello! I've tried minimock recently, and it's a great tool. However, when I tried it, I got stuck for a while because I accidentally specified a struct implementing an interface instead of the interface itself.

Consider the following source code:

package main

type Formatter interface {
	Format(string, ...interface{}) string
}

type FormatterImpl struct {
}

func (i *FormatterImpl) Format(string, ...interface{}) string {
	return ""
}

If instead of minimock -i Formatter you run minimock -i FormatterImpl, no output is generated, which is expected. However, there is no error - it just exits silently. Since specifying a struct instead of interface is always wrong, I propose throwing an error in this case - it's a small patch, but it might save some of the user's time.