Comment 1 for bug 1646644

Here's the Traceback:

Traceback (most recent call last):
  File "/usr/lib/update-notifier/package-data-downloader", line 264, in process_download_requests
    result = subprocess.call(command)
  File "/usr/lib/python3.5/subprocess.py", line 557, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.5/subprocess.py", line 947, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.5/subprocess.py", line 1551, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: '/usr/lib/flashplugin-installer/install_plugin'

We should probably confirm that command[0] actually exists before trying to run it.