Remove System#exit call from BitsquareApp#stop

Calling System#exit short-circuits normal error reporting, meaning that
when an exception is called within the scope of executing the
Application's #start method, the message

    Exception in Application start method

is printed to the console, but without a stack trace. Removing the
System#exit call allows JavaFX's LauncherImpl to continue processing the
exception, ultimately printing out the stack trace as expected.

src/main/java/io/bitsquare/app/gui/BitsquareApp.java

@@ -148,7 +148,6 @@ public class BitsquareApp extends Application {
     @Override
     public void stop() {
         bitsquareAppModule.close(injector);
-        System.exit(0);
     }