adb: add help text for USB permission errors.

The current permission messages can be confusing for users who don't
know about udev and USB access permissions. This CL adds some checks to
try to identify common udev problems, and adds a link to online
documentation.

Example messages:

1) adb server is in plugdev group but access is still denied:
$ adb devices
List of devices attached
082f59270073e1e3  no permissions (verify udev rules); see [developer.android.com/tools/device.html]

2) plugdev group exists but adb server is not in it:
$ adb shell
error: USB permission failure: udev requires plugdev group membership.
See [developer.android.com/tools/device.html] for more information.

3) plugdev group does not exist:
$ adb shell
error: USB permission failure.
See [developer.android.com/tools/device.html] for more information.

Bug: http://b/25777880
Change-Id: I536565adc12ab657c75151309795674181205db0