Improve reporting of invalid repeating groups

Steve,

I ran into a problem with repeating groups that I think this patch would help. A counterparty was sending a message containing a repeating group that was out of order (the first field was actually second) and QF/J threw an NPE in parseGroup() when it tried to add the first field it found to the not-yet-constructed group. This patch changes parseGroup() to throw InvalidMessage when the first field found is out of order, and
fromString() to rethrow that.

John Hensley