gdb.debug not so obvious error msg

[disconnect3d]

When the gdb.debug fails to launch binary under gdbserver it shows a not-super-obvious error. This might happen for example when one sets LD_PRELOAD to load different libc library.

Example

NOTE: I had to run sudo apt-get update && sudo apt-get install gdbserver first.
NOTE2: I have used /bin/bash as LD_PRELOAD but that could be a .so library not compatible with the binary (e.g. wrong arch).

pwntools@a0b508e4b82d:~$ python -c 'from pwn import *; gdb.debug("/bin/ls", env={"LD_PRELOAD": "/bin/bash"})'
[+] Starting local process '/usr/bin/gdbserver': pid 254
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/pwnlib/context/__init__.py", line 1349, in setter
    return function(*a, **kw)
  File "/usr/local/lib/python2.7/dist-packages/pwnlib/gdb.py", line 396, in debug
    port = _gdbserver_port(gdbserver, ssh)
  File "/usr/local/lib/python2.7/dist-packages/pwnlib/gdb.py", line 233, in _gdbserver_port
    gdbserver.pid   = int(process_created.split()[-1], 0)
ValueError: invalid literal for int() with base 0: 'ignored.'
[*] Stopped process '/bin/ls' (pid 254)
pwntools@a0b508e4b82d:~$ 

[zachriggle]

I'm not sure of any way we can really detect this in the generic case.

I'd be interested in a pull request if you have time <3

[zachriggle]

Closed with #1083