Implement GOSUB/GOTO x:@LABEL

SMILEBASIC/error.d

@@ -38,6 +38,14 @@ class SmileBasicError : Exception
         return message2;
     }
 }
+class InternalError : SmileBasicError
+{
+    this()
+    {
+        this.errnum = 1;
+        super("Internal Error");
+    }
+}
 class SyntaxError : SmileBasicError
 {
     this()
@@ -203,6 +211,14 @@ class SubscriptOutOfRange : SmileBasicError
         super("Subscript out of range(" ~ func ~ ":" ~ arg.to!string ~ ")");
     }
 }
+class IllegalSymbolString : SmileBasicError
+{
+    this()
+    {
+        this.errnum = 34;
+        super("Illegal symbol string");
+    }
+}
 class UsePRGEDITBeforeAnyPRGFunction : SmileBasicError
 {
     this(string func)