[xslt] Throw error if XPath doesn't match

[Andreas Sommer <AndiDog web de>]

Hi,

I want to know if it's possible to throw an error if the XPath "foo" of 
<xsl:value-of select="foobar" /> does not match anything in the source 
file. For example if the path is an optional element and I want to make 
sure that the XSL stylesheet only works if the element is given in the 
source file.

Cheers
Andreas