Add a getter for the Entry.err

[VadimKulagin]

Hello!

I want to implement my Formatter and for this I need access to the Entry.err field. But, unfortunately, this is not possible now, since this field is private.

I ended up adding a getter for this field:

// Err returns an internal field formatting error.
func (entry *Entry) Err() string {
	return entry.err
}

[VadimKulagin]

Are there any chances for merge?

[pashaosipyants]

@markphelps Still relevant enhancement, not stale. Any updates/thoughts?