type errors cause ugly error msgs

[David Graf]

A query like this

declare function local:foo() as empty-sequence()
 {
   <foo />
 };

 local:foo();

causes the following error msg because of a type error:

<?xml version="1.0" encoding="UTF-8"?>
</Users/wcandillon/tmp/test.xq>:3,3: type error [err:XPTY0004]: Cannot
treat [NodeXQType elementNode nametest=[uri: , local: foo]
content=[XQType ANY_TYPE_KIND*]] as [XQType NONE_KIND] when returning
the result of the function local:foo().; raised at
/Users/wcandillon/28msec/zorba/src/runtime/core/sequencetypes.cpp:557
=================================================
local:foo#0 <http://www.w3.org/2005/xquery-local-functions>
/Users/wcandillon/tmp/test.xq at line 6 column 1

This is very ugly und not understandable for someone who is not involved in the zorba internals. We need an error msg like this:

Stopped at line 1, column 17: [XPTY0004] Can't promote type xs:string to empty-sequence(): "abc".