image_to_usb.sh: warn before copying to non-existent /dev/sdX device.
image_to_usb.sh: warn before copying to non-existent /dev/sdX device.

Without this fix, when the script is invoked with (say) --to=/dev/sdk
but /dev/sdk is not a valid block device, we will interpret it as if the
user intended to copy the image to a file of the same name. However, it
is more likely that the user specified the wrong device name.  Hence,
this fix catches this type of error and will fail the operation with
a corresponding error message.

Also includes minor changes to printed messages.

BUG=None
TEST=Ran the script with different values of --to

Change-Id: Ie060ff2be77b8c065ba95f635455049e034d4bbc
Reviewed-on: https://gerrit.chromium.org/gerrit/15822
Reviewed-by: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>
Tested-by: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>
Commit-Ready: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>

BugRoger/scripts,fivethreeo/scripts,endocode/scripts,vmware/coreos-scripts,fivethreeo/scripts,endocode/scripts,andrejro/coreos--scripts,zhang0137/scripts,zhang0137/scripts,mischief/scripts,trnubo/scripts,vmware/coreos-scripts,crawford/scripts,glevand/coreos--scripts,marineam/coreos-scripts,nicescale/scripts,fivethreeo/scripts,sigma/coreos-scripts,andrejro/coreos--scripts,trnubo/scripts,marineam/coreos-scripts,mjg59/scripts,marineam/coreos-scripts,gtank/scripts,vmware/coreos-scripts,endocode/scripts,sigma/coreos-scripts,mjg59/scripts,mischief/scripts,crawford/scripts,trnubo/scripts,sigma/coreos-scripts,smilart/scripts,nicescale/scripts,bcwaldon/coreos-scripts,bcwaldon/coreos-scripts,smilart/scripts,nicescale/scripts,crawford/scripts,coreos/scripts,BugRoger/scripts,gtank/scripts,mischief/scripts,BugRoger/scripts,mjg59/scripts,zhang0137/scripts,glevand/coreos--scripts,andrejro/coreos--scripts,gtank/scripts,smilart/scripts,bcwaldon/coreos-scripts,coreos/scripts