Provide useful error and exit gracefully if app has no usable version

[homebysix]

I recently came across an app without either a CFBundleShortVersionString or CFBundleVersion key in its Info.plist file. Quickpkg understandably failed to package the app, but the provided traceback didn't make it obvious to me what the problem was.

% ./quickpkg ~/Desktop/PCoIPClient.app             
Traceback (most recent call last):
  File "./quickpkg", line 427, in <module>
    result = cmdexec(analyzecmd)
  File "./quickpkg", line 35, in cmdexec
    proc = subprocess.Popen(
  File "/Library/ManagedFrameworks/Python/Python3.framework/Versions/3.9/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/Library/ManagedFrameworks/Python/Python3.framework/Versions/3.9/lib/python3.9/subprocess.py", line 1754, in _execute_child
    self.pid = _posixsubprocess.fork_exec(
TypeError: expected str, bytes or os.PathLike object, not NoneType

This pull request adds an exit condition if no usable version key is found that should eliminate this confusion if anybody encounters the same problem I did.

% ./quickpkg ~/Desktop/PCoIPClient.app                                         
Application at path /var/folders/0n/svv6yz7d5qlf59695s39bnx40000gn/T/tmpu2zm2dyp/payload/PCoIPClient.app does not have a CFBundleShortVersionString or CFBundleVersion