Add error handling

greetings.go

@@ -1,10 +1,18 @@
 package greetings
 
-import "fmt"
+import (
+    "errors"
+    "fmt"
+)
 
 // Hello returns a greeting for the named person.
 func Hello(name string) string {
+    // If no name was given, return an error with a message.
+    if name == "" {
+        return "", errors.New("empty name")
+    }
+
     // Return a greeting that embeds the name in a message.
     message := fmt.Sprintf("Hi, %v. Welcome!", name)
-    return message
+    return message, nil
 }