Adjust expected error output for libxml2 changes

tests/general/bug-165.err

 runtime error: file ./bug-165.xsl line 6 element value-of
 Variable 'pStyle' has not been declared.
+XPath error : Undefined variable
 runtime error: file ./bug-165.xsl line 6 element value-of
 XPath evaluation returned no result.
 no result for ./../docs/bug-165.xml

tests/general/bug-180.err

 runtime error: file ./bug-180.xsl line 4 element copy-of
 Variable 'xxx' has not been declared.
+XPath error : Undefined variable
 no result for ./../docs/bug-180.xml

tests/general/bug-184.err

 runtime error: file ./bug-184.xsl line 5 element value-of
 Variable 'nonexistent' has not been declared.
+XPath error : Undefined variable
 runtime error: file ./bug-184.xsl line 5 element value-of
 XPath evaluation returned no result.
 no result for ./../docs/bug-184.xml

tests/general/bug-194.err

 runtime error
 Variable 'p' has not been declared.
+XPath error : Undefined variable
 runtime error: file ./bug-194.xsl line 4 element param
 Failed to evaluate the expression of variable 'p'.
@@ -3,5 +4,6 @@
 runtime error: file ./bug-194.xsl line 4 element param
 Failed to evaluate the expression of variable 'p'.
+XPath error : Undefined variable
 runtime error: file ./bug-194.xsl line 5 element param
 Failed to evaluate the expression of variable 'p2'.
 no result for ./../docs/bug-194.xml