Nicer error when running tmux exploit outside tmux

[peace-maker]

When you set the context.terminal to be tmux related but then try to run the exploit outside a tmux session, you'd get a cryptic ValueError while trying to parse the pid of the new pane in tmux.

Print a nicer reminder to start tmux first.

from pwn import *
context.terminal = ['tmux', 'splitw', '-h']
gdb.debug('/bin/cat').interactive()

[+] Starting local process '/usr/bin/gdbserver': pid 3401
[*] running in new terminal: ['/usr/bin/gdb', '-q', '/bin/cat', '-x', '/tmp/pwni46_dcye.gdb']
Traceback (most recent call last):
  File "/home/peace/dev/pwntools/blabla.py", line 4, in <module>
    gdb.debug('/bin/cat').interactive()
  File "/home/peace/dev/pwntools/pwnlib/context/__init__.py", line 1581, in setter
    return function(*a, **kw)
  File "/home/peace/dev/pwntools/pwnlib/gdb.py", line 582, in debug
    tmp = attach((host, port), exe=exe, gdbscript=gdbscript, ssh=ssh, sysroot=sysroot, api=api)
  File "/home/peace/dev/pwntools/pwnlib/context/__init__.py", line 1581, in setter
    return function(*a, **kw)
  File "/home/peace/dev/pwntools/pwnlib/gdb.py", line 1100, in attach
    gdb_pid = misc.run_in_new_terminal(cmd, preexec_fn = preexec_fn)
  File "/home/peace/dev/pwntools/pwnlib/util/misc.py", line 413, in run_in_new_terminal
    pid = int(out)
ValueError: invalid literal for int() with base 10: b''
[*] Stopped process '/bin/cat' (pid 3404)