Handle missing device more gracefully

[coldfix]

Calling udiskie-info with an invalid device (and other valid ones), no info is printed and an exception traceback is shown instead:

However, it breaks when when searching for /dev/loop

myuser@mycomputer:~$ udiskie-info -o 'id_type' /dev/loop*
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/udiskie/cli.py", line 289, in _start_async_tasks
    results = yield self._init()
  File "/usr/lib/python3/dist-packages/udiskie/cli.py", line 724, in _init
    devices = [self.udisks.find(path) for path in options['<device>']]
  File "/usr/lib/python3/dist-packages/udiskie/cli.py", line 724, in <listcomp>
    devices = [self.udisks.find(path) for path in options['<device>']]
  File "/usr/lib/python3/dist-packages/udiskie/udisks2.py", line 584, in find
    raise ValueError(_('no device found owning "{0}"', path))
ValueError: master device was not found for "/dev/loop-control"

Originally posted by @mixpc in #177 (comment)

This should be handled more gracefully, show a "invalid device: {...}" message on stderr, but print the rest of the information nevertheless.

Opening this here as separate issue so I don't forget about it.