restore support for previous sdk versions

After commits https://github.com/clojure-android/lein-droid/commit/448ea332bc009c8ac81d47e9939e252592a11056 and https://github.com/clojure-android/lein-droid/commit/96a8fd82a2cd00bd36a1d83c341074bad5425b89 lein-droid does not seem to support SDK versions without build-tools so anyone who, like me, haven't upgraded yet will be affected. Therefore I propose this modification that restores previous behavior when build-tools directory does not exist and tries to warn user about possible cause of missing tool error.