If the systemId is not for us to handle, we delegate to our super
class, at leasts that's the basic idea. However, the code below
needs to be more complicated.
Due to inexplicable voodoo, the original resolveEntity method in
org.xml.sax.helpers.DefaultHandler declares throwing an IOException,
whereas the org.xml.sax.helpers.DefaultHandler class included in
JDK 1.4 masks this exception. In JDK 1.5, the IOException has been
put back...
In order to compile under JDK 1.4, we are forced to mask the IOException
as well. Since its signatures varies, we cannot call our super class'
resolveEntity method. We are forced to implement the default behavior
ourselves, which in this case, is just returning null.