Add StackOver/UnderFlow

SMILEBASIC/VM.d

@@ -176,8 +176,7 @@ class VM
     {
         if(stacki >= this.stack.length)
         {
-            writeln("Stack OF");
-            readln();
+            throw new StackOverFlow();
         }
         stack[stacki++] = value;
     }
@@ -193,8 +192,7 @@ class VM
     {
         if(stacki <= bp)
         {
-            writeln("Stack underflow");
-            readln();
+            throw new StackUnderFlow();
         }
         value = stack[--stacki];
     }
@@ -202,8 +200,7 @@ class VM
     {
         if(stacki <= bp)
         {
-            writeln("Stack underflow");
-            readln();
+            throw new StackUnderFlow();
         }
         --stacki;
     }

SMILEBASIC/error.d

@@ -56,6 +56,22 @@ class IllegalFunctionCall : SmileBasicError
         super("Illegal function call(" ~ func ~ ")");
     }
 }
+class StackOverFlow : SmileBasicError
+{
+    this()
+    {
+        this.errnum = 5;
+        super("Stack overflow");
+    }
+}
+class StackUnderFlow : SmileBasicError
+{
+    this()
+    {
+        this.errnum = 6;
+        super("Stack underflow");
+    }
+}
 class TypeMismatch : SmileBasicError
 {
     this()