XPath: Assume that every function can fail and return 0
XPath: Assume that every function can fail and return 0

Propagate the failure to the caller manually. This is a first step to
parser structure that does not depend on exceptions or longjmp for error
handling (and thus matches the XML parser). To preserve semantics we'll
have to convert error code to exception later.

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml