_register method of TM class in Shared/DC/ZRDB/TM.py hides all exceptions.

This makes descendants of this class prone to get stuck in a "registered" 
state withtout any possibility of them to get commited.

To reproduce the problem:
 - create a class inheriting from TM, and define a method calling register.
   Add logs to abort and commit method to track transaction end.
 - create a default error page which triggers a call to the method created
   above. (this is equivalent to accessing some database from the error page,
   for example)
   Call it twice.
   Most realistic case is using an instance surviving the transaction (a
   global, or a persistant object)
 - tigger an error in TPC (a raise in vote is the most realistic case) and get
   the error page to render. The most obvious breakage I could see is when
   trying to undo an non-undoable transaction (modify a script twice, undo the
   oldest without undoing the newest)

Here is what happens:
 - first transaction (the "undo" in my example) raises in TPC, transaction is
   marked as failed
 - error message gets rendered in the same transaction (that's a ZPublisher
   bug, but I think the problem "root" is hiding the failure)
 - error message tries to register itself to transaction manager:
   First try gets an exception (hidden in TM._register), but in the process
   transaction's "_adapters" dict was modified. TM subclass instance is not
   marked as registered.
   Second try, now there is no exception raised, because transaction's "join"
   is not called (because of _adapters content). Now the TM subclass instance
   is marked as registered, but unknown to transaction (_adapters will not be
   used to commit).

The variant with just one call to TM._register "only" causes the instance to 
be commited at next transaction using it.

Is there any reason why TM._register is hiding exceptions ?

Vincent Pelletier
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