xsltproc should return an error code if xinclude fails

When running xsltproc with the --xinclude option and if the included file
contains parse errors, then xsltproc exits with a success return code (0)
rather than an error code.  This is despite the fact that parser error
messages are printed out.
* xsltproc/xsltproc.c: check xinclude processing function return code,
  fail with error 6 if it went wrong.