Update docs and tests around cli.Exit

Some of the docs were lacking or incomplete concerning how to properly
use cli.Exit, cli.HandleExitCoder, and the ExitErrHandler field on cli.App.
This change aims to clarify the usage of those pieces.

I also noticed that we have two identical functions now: cli.Exit and
cli.NewExitError. Since the latter seems to be more recent and less well
documented, I went ahead and marked it as deprecated so that we can keep
our public interface small.

Also added a missing test case for behavior that's been around a while
but was not documented or tested.

Related: #1089

app.go

@@ -76,8 +76,9 @@ type App struct {
 	Writer io.Writer
 	// ErrWriter writes error output
 	ErrWriter io.Writer
-	// Execute this function to handle ExitErrors. If not provided, HandleExitCoder is provided to
-	// function as a default, so this is optional.
+	// ExitErrHandler processes any error encountered while running an App before
+	// it is returned to the caller. If no function is provided, HandleExitCoder
+	// is used as the default behavior.
 	ExitErrHandler ExitErrHandlerFunc
 	// Other custom info
 	Metadata map[string]interface{}