Predeclare object and error as import prefixes

Like xml. Part of #413.

lang/spec.html

@@ -7125,9 +7125,10 @@ <h3>Import declaration</h3>
 symbol space used by other declarations.
 </p>
 <p>
-A module prefix of <code>xml</code> is predeclared as referring to the
-<code>lang.xml</code> lang library module, but this can be overridden by an
-import-decl.
+A module prefix of <code><var>t</var></code>, where <code><var>t</var></code> is
+one of <code>error</code>, <code>object</code> or <code>xml</code>, is
+predeclared as referring to the <code>lang.<var>t</var></code> lang library
+module, but this can be overridden by an import-decl.
 </p>
 <p>
 It is an error for a module to directly or indirectly import itself. In other