Add missing scripts/ex/error-subtype.js

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scripts/ex/error-subtype.js: Add file.

scripts/ex/error-subtype.js

@@ -0,0 +1,15 @@
+const MyError = Class( 'MyError' )
+    .extend( Error, {} );
+
+const e = MyError( 'Foo' );
+e.message;  // Foo
+e.name;     // MyError
+
+// -- if supported by environment --
+e.stack;         // stack beginning at caller
+e.fileName;      // caller filename
+e.lineNumber;    // caller line number
+e.columnNumber;  // caller column number
+
+// general case
+throw MyError( 'Foo' );