disassemble command: fix error message when disassembly fails

We were printing the start_addr field, which is not correct, as in this
branch we are processing the memory described by cur_range. Print that
instead.

Ideally, in particular this case, the error message would also say
something about not being able to disassemble due to not having found
the module from the core file, but that is not easy to do right now, so
I'm leaving that for another time.

llvm-svn: 370898