[xsl] Signature of a function that may throw an error

["Christophe Marchand cmarchand@xxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Hello,

I have a function that must return a xs:string, or raise an error, if 
result value can not be computed, or is empty-sequence.

Here is function :

 B  <xsl:function name="conf:getConfVar" as="xs:string">
 B B B  <xsl:param name="conf" as="element(els-conf:conf)"/>
 B B B  <xsl:param name="varName" as="xs:string"/>
 B B B  <xsl:sequence select="
 B B B B B  if (exists($conf/els-conf:variable[@name=$varName]))
 B B B B B  then $conf/els-conf:variable[@name=$varName]/@value
 B B B B B  else (
 B B B B B B B  error(
 B B B B B B B B B  $conf:ERROR_MISSING_ENTRY,
 B B B B B B B B B  concat('Entry ',$varName,' is missing in environment 
configuration')
 B B B B B B B  )
 B B B B B  )
 B B B B B  "/>
 B  </xsl:function>

When a variable does not exist in $conf, I expect an error being raised, 
and expect to catch it in a xsl:try / xsl:catch.

But, in this case, I get this :

XTTE0780: Conditional expression: The condition is not satisfied, so an 
empty sequence is returned, but this is not allowed as the result of 
call to conf:getConfVar
in xsl:sequence/@select ....

  * I understand that error() function never return a value, so return
    value does not satisfies xsl:function/@as constrint. Is it correct ?
  * How could I write this, as I want the return type being xs:string,
    and not xs:string?, and I want my error to be raised when entry is
    missing ?

Best regards,
Christophe