Updater: Do not throw Errors when parsing the update sites

A recent, ill-considered change to the Fiji wiki page listing the
update sites broke the updater because parse errors would throw
uncaught Errors rather than IOExceptions.

Reported by Niko Ehrenfeuchter.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: make update site list parser even more robust

MediaWiki does not allow you to add an empty line at the end of the
page source, therefore the parser broke when we moved the comment
how to add update sites to the list to the top.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: prepare the update site list parser to a different column
order

We had a broken updater for a day due to heavy changes in the layout
of http://fiji.sc/List_of_update_sites. Partially, the changes made
sense, even if they broke the updater. By preparing the updater
for those changes, we can make those changes again, after the vast
majority will have a new version of the updater already.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'updater-hotfix'

Due to extensive layout changes in the Fiji wiki page listing the
available update sites, the updater was broken. We reverted those
changes for now, but it is much better to prepare the updater
for future breakages of the same kind, and let it continue -- ignoring
the unexpected format.

Thanks, Niko Ehrenfeuchter, for bringing this problem to our attention.

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