[Ada] Improved error recovery for positional box

[Arnaud Charlet <charlet at adacore dot com>]

This patch improves the error recovery for a box used in positional
notation in an aggregate (named notation is required for this case).

The following test is shown twice

     --  compiled with -gnat05

     1. package badbox is
     2.    x : string := (<>, 'a', 'b');
                          |
        >>> (Ada 2005) box only allowed with named notation

     3.    y : string := ('a', <>, 'b');
                               |
        >>> (Ada 2005) box only allowed with named notation

     4. end badbox;

     --  compiled with -gnat95

     1. package badbox is
     2.    x : string := (<>, 'a', 'b');
                          |
        >>> box in aggregate is an Ada 2005 extension
        >>> (Ada 2005) box only allowed with named notation

     3.    y : string := ('a', <>, 'b');
                               |
        >>> box in aggregate is an Ada 2005 extension
        >>> (Ada 2005) box only allowed with named notation

     4. end badbox;

Tested on x86_64-pc-linux-gnu, committed on trunk

2010-09-09  Robert Dewar  <dewar@adacore.com>

	* par-ch4.adb (Box_Error): New procedure.

Attachment: difs
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