#171 MBeanProxy exception handling changed

[Juha Lindfors]

The declared exceptions for the MBeanProxy create and
get methods have changed -- instead of declaring 4 to 6
different JMX exceptions the proxy throws just one
MBeanProxyCreationException. Hopefully this saves some
unnecessary typing effort.

This went into the 1.1 dev branch, will be released as
JBossMX 1.2