Fix weblient crash and handle broken image links in public shares correctly

[dpwrussell]

This fixes the webclient crash introduced in #3124 when viewing shares with broken image links. Previously to this they were rendered as a special broken link, now it doesn't get to that point because of a webclient crash.

Testing Instructions

• Create a share with some images
• Delete one of those images
• Try and load the share, a broken link icon should be displayed instead of a webclient crash.

[atarkowska]

Please merge https://github.com/dpwrussell/openmicroscopy/pull/4

[atarkowska]

Thanks for finding bugs, next time please report it straight to the PR when was still open ;-)

[dpwrussell]

@aleksandra-tarkowska Thanks, forgot about that.

Edited description:

It also now fixes how the broken links display in the tree and removes the old template code from the tree which was not used any more.

[pwalczysko]

The workflow as described in the header of this PR is fine.

I have 2 issues (for another PR ?):

• when the broken link image thumbnail is double clicked, an 404 page comes up (see screenshot)
• when an image is removed from the share using selection in the tree and then Hitting the Delete Key on keyboard, the image vanishes from the tree but not from the central pane, this stays so even after reload

[dpwrussell]

@pwalczysko Thanks. Yes, those are both issues with the current implementation as well so another PR would be best. The latter is fixed #3141 already.

[atarkowska]

--rebased-to #3172