Improve udev failure diagnostics.

A couple of folks had trouble understanding the existing message.

Before:

  8XV7N15917000596	no permissions (udev requires plugdev group membership); see [http://developer.android.com/tools/device.html]

After:

  8XV7N15917000596	no permissions (user buttmunch is not in the plugdev group); see [http://developer.android.com/tools/device.html]

This also fixes a libusb regression where we wouldn't show anything for
devices where we don't have permissions.

Bug: http://b/37707122
Test: ran "adb devices" as user buttmunch
Change-Id: I2fcd735ff4178145432b532a6e4dc8c93b2743fd