ImageJ launcher: fail gracefully when Retrotranslator was not found
ImageJ launcher: fail gracefully when Retrotranslator was not found

Noticed by Curtis Rueden.

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

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