Define format for error reason string

Fixes #227.

lang/spec.html

@@ -1187,6 +1187,22 @@ <h4>Error</h4>
 stack</li>
 </ul>
 <p>
+A <em>module-qualified reason</em> string is a string that has the form
+</p>
+<pre>
+   <code>{</code><var>org-name</var><code>/</code><var>module-name</var><code>}</code><var>identifier</var>
+</pre>
+<p>
+where <code><var>org-name</var></code>, <code><var>module-name</var></code> and
+<code><var>identifier</var></code> are as defined by the grammar in this
+specification, but with no whitespace allowed between tokens. Any reason string
+that starts with a <code>{</code> should be a module-qualified reason string.
+Any error value that is constructed as the associated value of a panic will use
+a module-qualified reason with an org-name of <code>ballerina</code> and a
+module-name that starts with <code>lang.</code>, as will any error value
+constructed by a function in the lang library.
+</p>
+<p>
 The shape of an error value consists of the shape of the reason and the shape of
 the detail; the stack trace is not part of the shape. A type descriptor
 error&lt;<var>r</var>, <var>d</var>&gt; contains an error shape if <var>r</var>