Importer: add various sanity checks for data from project files

This commit will make the import complain in a clearer way of information is
missing from the project files. This is related to issue #579.
Importer: handle IOErrors with URLs more gracefully

This change will make it possible to also allow stacks if the image data isn't
reachable. Like for unreadable data (on reachable servers), URLs of these stacks
will be highlighted on the overview page. This relates to issue #579.
Fix example project file in importer documentation

It was lacking some required keywords and the opacity was in the wrong range.
Importer: fix typos