Launcher: skip the class launcher when its .jar file is not present
Launcher: skip the class launcher when its .jar file is not present

It makes little sense to call the -- otherwise useful -- ClassLauncher
class when the ij-launcher .jar file is not present, as it is pretty
unlikely that said class will be found in that case.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

TehSAUCE/imagej,biovoxxel/imagej,TehSAUCE/imagej,biovoxxel/imagej,TehSAUCE/imagej,biovoxxel/imagej