Improve error message UI for MediaViewer
completed by: m4tx
mentors: Gergő Tisza
MediaViewer displays images with associated metadata on top of wiki pages. When the image cannot load, it displays an error message which is currently rather ugly. Use the mockup from T77272 to improve its look. One of the comments there has a code example for simulating loading errors.
Ignore the part about the metadata; that's harder to do and could be improved UI-wise.
Leave out the "retry" link in the error message; right now that functionality is missing. The other link should point to https://phabricator.wikimedia.org/maniphest/task/create/?projects=PHID-PROJ-cabyqp5sf4hyvauln3sq .
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