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

[Eli Zaretskii]

> Date: Thu, 16 Apr 2009 12:06:54 -0700
> From: Joel Brobecker <brobecker@adacore.com>
> Cc: gdb-patches@sourceware.org
> 
> > But an invalid address can sometimes (accidentally) be readable,
> > right?  What will we print then?
> 
> It looks like this "(bad)" is printed by opcode, not GDB.
> So we should get your "(bad)" output if the address is valid
> but points to an invalid instruction.

OK, but your suggested message text is too long, I think.  Something
shorter, like "(address unreadable)" is better, IMO.