[Ada] Bootstrapping mainline GNAT fails

[Robert Dewar]

<<On the other hand, if exceptions are used for non-fatal errors too,
then this won't work, unless you wanted to eliminate all such uses
(which would be a lot more effort).
>>

Exceptions are used for non-fatal errors, and also for dealing with
malformed trees. There are also places, like this one, where code is
protected with an exception handler so that if there is a bug in the
code it is harmless.

Usually of course we would not expect exceptions to be raised in the
absence of errors.