Fix_Updater.js: allow setting ij.dir from versioned ij.jar files

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix_Updater.js: IJ.error() already stops the script

... therefore we do not have to throw an exception, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Allow Fix_Updater.js to be started from unpacked .class files

When starting ImageJA via 'mvn exec:java', it is started from the
classes in target/classes/, not a .jar file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>