(Ada) provide the exception message when hitting an exception catchpoint

(Ada) provide the exception message when hitting an exception catchpoint

This patch enhances the debugger to print the exception message, when
available, as part of an exception catchpoint hit notification (both
GDB/CLI and GDB/MI). For instance, with the following code...

    procedure A is
    begin
       raise Constraint_Error with "hello world";
    end A;

... instead of printing...

    Catchpoint 1, CONSTRAINT_ERROR at 0x000000000040245c in a () at a.adb:3

... it now prints:

    Catchpoint 1, CONSTRAINT_ERROR (hello world) at 0x000000000040245c in a ()
                                   ^^^^^^^^^^^^^

This enhancement requires runtime support. If not present, the debugger
just behaves as before.

In GDB/MI mode, if the exception message is available, it is provided
as an extra field named "exception-message" in the catchpoint notification:

    *stopped,bkptno="1",[...],exception-name="CONSTRAINT_ERROR",
       exception-message="hello world",[...]

gdb/ChangeLog:

        * ada-lang.c (ada_exception_message_1, ada_exception_message):
        New functions.
        (print_it_exception): If available, display the exception
        message as well.
        * NEWS: Document new feature.

gdb/doc/ChangeLog:

        * gdb.texinfo (GDB/MI Ada Exception Information): Document
        new "exception-message" field.

gdb/testsuite/ChangeLog:

        * gdb.ada/catch_ex.exp, gdb.ada/mi_catch_ex.exp,
        gdb.ada/mi_ex_cond.exp: Accept optional exception message in
        when hitting an exception catchpoint.