Fall back to the remote updater if ImageJ cannot be started

This code moved from the Fiji Updater (relicensed to BSD by yours
truly, the original author of said code).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Avoid falling back to updater in batch mode

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fall back to remote Javascript to fall back to updater

In case Javascript is not available, let's get js.jar remotely, too

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a test drive for the new remote updater feature

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
When falling back to the remote updater, show the cause

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