improve Ada exception catchpoint MI notification

This rewrites the code generating the Ada exception catchpoint hit
notification for both the GDB/MI case as well as the non-MI case,
by using the relevant ui_out_* functions to generate the output.
the MI notifications for Ada exception catchpoints now include
the stop reason, and the breakpoint "disp", much like other breakpoint
events do.  It also introduces a new field "exception-name" for
exception catchpoints (excluding "failed assertion catchpoints,
where we just want to know that it was a failed assertion).

gdb/ChangeLog:

        * breakpoint.h (bpdisp_text): Add declaration.
        * breakpoint.c (bpdisp_text): Make non-static.
        * ada-lang.c: #include "mi/mi-common.h".
        (print_it_exception): Rewrite to improve GDB/MI output.

gdb/doc/ChangeLog:

        * gdb.texinfo (GDB/MI Ada Exception Information): Document
        the "exception-name" field in the *stopped async record.

gdb/testsuite/ChangeLog:

        * gdb.ada/mi_catch_ex: New testcase.

gdb/breakpoint.c

@@ -251,7 +251,7 @@ breakpoint_commands (struct breakpoint *b)
 
 static int breakpoint_proceeded;
 
-static const char *
+const char *
 bpdisp_text (enum bpdisp disp)
 {
   /* NOTE: the following values are a part of MI protocol and