Stacktrace when session does not exist

[whotwagner]

Currently a stacktrace occurs if a meterpreter-session is defined that does not exist:

Traceback (most recent call last):
  File "/home/aecid/penpal/venv/bin/penpal", line 8, in <module>
    sys.exit(main())
  File "/home/aecid/penpal/venv/lib/python3.10/site-packages/penpal/__main__.py", line 78, in main
    sys.exit(hacky.main())
  File "/home/aecid/penpal/venv/lib/python3.10/site-packages/penpal/penpal.py", line 109, in main
    self.msfsession.run(command)
  File "/home/aecid/penpal/venv/lib/python3.10/site-packages/penpal/baseexecutor.py", line 87, in run
    self.exec(command)
  File "/home/aecid/penpal/venv/lib/python3.10/site-packages/penpal/baseexecutor.py", line 98, in exec
    result = self._exec_cmd(command)
  File "/home/aecid/penpal/venv/lib/python3.10/site-packages/penpal/msfsessionexecutor.py", line 42, in _exec_cmd
    session_id = self.sessionstore.get_session_by_name(command.session, self.msf.sessions)
  File "/home/aecid/penpal/venv/lib/python3.10/site-packages/penpal/msfsessionstore.py", line 17, in get_session_by_name
    if v['exploit_uuid'] == self.sessions[name]:
KeyError: 'foothold'

This Error needs to be caught and handled properly