added db function to see if user exists

db.go

@@ -312,3 +312,29 @@ func (db *DB) addUser(email string) (string, error) {
 	)
 	return tok, err
 }
+
+func (db *DB) hasUser(email string) error {
+	result, err := db.conn.Query(
+		"SELECT EXISTS(SELECT 1 FROM users WHERE email = ? LIMIT 1)",
+		email,
+	)
+	if err != nil {
+		return verrors.HTTP{
+			Message: fmt.Sprintf("could not find requested user's email: %q: %v", email, err),
+			Code:    http.StatusInternalServerError,
+		}
+	}
+	var exists string
+	for result.Next() {
+		if err := result.Scan(&exists); err != nil {
+			log.Fatal(err)
+		}
+	}
+	if exists == "0" {
+		return verrors.HTTP{
+			Message: fmt.Sprintf("could not find requested user's email: %q: %v", email, err),
+			Code:    http.StatusInternalServerError,
+		}
+	}
+	return nil
+}