[errors] Make MultiError mutable

[prateek]

In an effort to keep errors.MultiErr immutable, we have the following for the Add method:

// Add adds an error returns a new MultiError object.
func (e MultiError) Add(err error) MultiError {
	if err == nil {
		return e
	}
	me := e
	if me.err == nil {
		me.err = err
		return me
	}
	me.errors = append(me.errors, me.err)
	me.err = err
	return me
}

There's a couple of issues with this:

• Almost every time someone unfamiliar with the API uses it, they mess up.
• We never need to keep the original value around, so we always use this like: mulitErr = multiErr.Add(...)

We should just change the signature to make it apparent the method mutates the underlying struct: func (e *MultiError) Add(err error) { ... }