[lang] Make the JDK and Xtext validation messages more clear.

Signed-off-by: Stéphane Galland <galland@arakhne.org>
[lang] Get the minimal JDK and Xtext versions from Maven.

see #256

Signed-off-by: Stéphane Galland <galland@arakhne.org>
[lang] Output an error when System.exit is called.

see #259

Signed-off-by: Stéphane Galland <galland@arakhne.org>
[lang] Add warning on the use of the println function and the System stream.

see #232

Signed-off-by: Stéphane Galland <galland@arakhne.org>
[lang] Apply code style.

Signed-off-by: Stéphane Galland <galland@arakhne.org>