#196 Unreported bad namespace in exclude-result-prefixes

[Michael Kay]

If the exclude-result-prefixes or extension-element-
prefixes attribute of an xsl:stylesheet element in an
included or imported stylesheet includes a namespace
prefix that hasn't been declared, no error is reported
and all the namespaces in that attribute are ignored.

This bug is present in all previous releases (including
6.5.3).

Source code fixed.