Change run-webkit-tests to handle 'no devices found' specially.

And generally clean up the exit_code logic so that we can pass back more
Android-specific exit codes in the future.

This change depends on https://codereview.chromium.org/27197012/ .

R=ojan@chromium.org, peter@chromium.org
BUG=

Review URL: https://codereview.chromium.org/27636002

git-svn-id: svn://svn.chromium.org/blink/trunk@159917 bbb929c8-8fbe-4397-9dbb-9b2b20218538