java FFI: display a clean error when java module is not imported

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Java FFI: enable inserting additionnal Java files to a project

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Java FFI: javac is always called on local files

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: add a test for the extra_java_files phase

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: Include extra Java files in a project

Will be used to declare implement new Activities and sub classes like Listeners.

Pull-Request: #496
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Frédéric Vachon <fredvac@gmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>