Added error message (for too-large array)

[dewar@gnat.com]

<<Interesting; what is the rationale for waiting until runtime to complain
about things which can reasonably and consistently be diagnosed at compile
time?
>>

The set of things that make an Ada program illegal is very well defined
by the standard. There is nothing illegal by Ada standards about these
declarations. The only limitation is that there is not enough storage to
actually allocate a gigantic array, hence the storage error if you actually
try. Of course warnings are always allowed.