gdb: improve error reporting from the disassembler

gdb: improve error reporting from the disassembler

If the libopcodes disassembler returns a negative value then this
indicates that the disassembly failed for some reason.  In disas.c, in
the function gdb_disassembler::print_insn we can see how this is
handled; when we get a negative value back, we call the memory_error
function, which throws an exception.

The problem here is that the address used in the memory_error call is
gdb_disassembler::m_err_memaddr, which is set in
gdb_disassembler::dis_asm_memory_error, which is called from within
the libopcodes disassembler through the
disassembler_info::memory_error_func callback.

However, for this to work correctly, every time the libopcodes
disassembler returns a negative value, the libopcodes disassembler
must have first called the memory_error_func callback.

My first plan was to make m_err_memaddr a gdb::optional, and assert
that it always had a value prior to calling memory_error, however, a
quick look in opcodes/*-dis.c shows that there _are_ cases where a
negative value is returned without first calling the memory_error_func
callback, for example in arc-dis.c and cris-dis.c.

Now, I think that a good argument can be made that these disassemblers
must therefore be broken, except for the case where we can't read
memory, we should always be able to disassemble the memory contents to
_something_, even if it's just '.word 0x....'.  However, I certainly
don't plan to go and fix all of the disassemblers.

What I do propose to do then, is make m_err_memaddr a gdb::optional,
but now, instead of always calling memory_error, I add a new path
which just calls error complaining about an unknown error.  This new
path is only used if m_err_memaddr doesn't have a value (indicating
that the memory_error_func callback was not called).

To test this I just augmented one of the disassemblers to always
return -1, before this patch I see this:

  Dump of assembler code for function main:
     0x000101aa <+0>:	Cannot access memory at address 0x0

And after this commit I now see:

  Dump of assembler code for function main:
     0x000101aa <+0>:	unknown disassembler error (error = -1)

This doesn't really help much, but that's because there's no way to
report non memory errors out of the disasembler, because, it was not
expected that the disassembler would ever report non memory errors.