Throw a runtime error on division by zero

src/com/craftinginterpreters/lox/Interpreter.java

@@ -110,10 +110,23 @@ class Interpreter implements Expr.Visitor<Object> {
                     return (String) left + (String) right;
                 }
 
+                if (left instanceof String) {
+                    return (String) left + stringify(right);
+                }
+
+                if (right instanceof String) {
+                    return stringify(left) + (String) right;
+                }
+
                 throw new RuntimeError(expr.operator, "Operands must be two numbers or two strings.");
-            case SLASH:
+        case SLASH: {
                 checkNumberOperands(expr.operator, left, right);
-                return (double) left / (double) right;
+                double rhs = (double) right;
+                if (rhs == 0) {
+                    throw new RuntimeError(expr.operator, "Division by zero.");
+                }
+                return (double) left / rhs;
+        }
             case STAR:
                 checkNumberOperands(expr.operator, left, right);
                 return (double) left * (double) right;