[RFA] print error message if (auto) disassembly failed

[Joel Brobecker]

> 2009-04-16  Joel Brobecker  <brobecker@adacore.com>
> 
>         * stack.c (do_gdb_disassembly): Print the exception message if an
>         error was thrown while trying to perform the disassembly.

I checked this one in.

-- 
Joel