Better error message for jail_prober.py cli usage

Curerently sys.argv is accessed without checks, resulting in an
IndexError:
```
Traceback (most recent call last):
  File "/home/rusty-snake/Projects/firejail/contrib/jail_prober.py", line 205, in <module>
    main()
  File "/home/rusty-snake/Projects/firejail/contrib/jail_prober.py", line 170, in main
    profile_path = sys.argv[1]
IndexError: list index out of range
```

This commit catches this IndexError and prints a more helpfull message
instaed:
```
USAGE: jail_prober.py <PROFILE-PATH> <PROGRAM>
```

contrib/jail_prober.py

@@ -164,8 +164,12 @@ def run_firejail(program, all_args):
 
 
 def main():
- profile_path = sys.argv[1]
- program = sys.argv[2]
+ try:
+ profile_path = sys.argv[1]
+ program = sys.argv[2]
+ except IndexError:
+ print('USAGE: jail_prober.py <PROFILE-PATH> <PROGRAM>')
+ sys.exit()
  # Quick error check and extract arguments
  check_params(profile_path)
  profile = get_args(profile_path)