vcl: improve ErrCode std::ostream pretty-printer

Let's say you specify a non-existing export filter during conversion.

Old output:

Error: Please verify input parameters... (SfxBaseModel::impl_store <file:///test.html> failed: 0x81a(Error Area:Io Class:Parameter Code:2074))

New output:

Error: Please verify input parameters... (SfxBaseModel::impl_store <file:///test.html> failed: 0x81a(Error Area:Io Class:Parameter Code:26))

The later (at least) allows going to include/vcl/errcode.hxx, searching
for Io.*Parameter.*26, and leads to ERRCODE_IO_INVALIDPARAMETER.

Change-Id: Iecfcfa4e805147131fde73cc0b89fa38f6ea61d4
Reviewed-on: https://gerrit.libreoffice.org/50421Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

vcl/source/helper/errcode.cxx

@@ -138,7 +138,7 @@ VCL_DLLPUBLIC std::ostream& operator<<(std::ostream& os, const ErrCode& err)
                 os << "Compiler";
                 break;
         }
-        os << " Code:" << OUString::number(err.GetRest());
+        os << " Code:" << OUString::number(err.GetRest() & 0xff);
     }
     os << ")";
     return os;