Fix the schema validation API

[zhouhao3]

Fix #686
In image-tools, warn is handled as follows:

if err := validate(content); err != nil {
	for _, e := range err.Warns {
			logrus.Warn(e)
	}
...
}

Signed-off-by: Zhou Hao [email protected]

[zhouhao3]

This is a preliminary idea, and if it works, I will continue.

I also have the idea of using logrus.warnf to handle warnings directly.
@stevvooe @vbatts @wking @xiekeyang PTAL

[vbatts]

house keeping: i think this is okay, but i'm not sure who all this will break with such a change. This makes me inclined to do nothing.

[vbatts]

cleaning up as there is no further conversation