bootstrap.js: If the variable 'cause' is injected, show it

We use bootstrap.js not only as a script to initialize a fresh
ImageJ.app/Fiji.app, but also to fall back in case of a non-working
setup. In the latter case, it would be nice to have some indication as
to the cause...

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