ui: Make error screen less verbose

This allows seeing the error message on smaller screens.
ui: Fix erroneous oops on loading images

Setting an empty src on the img of the viewer apparently triggers an
error on some browsers (at least iPhone and iPod touch running iOS 7),
this commit swaps the empty src for an empty pixel when clearing the
viewer image.
ui: Move viewer toolbar popup logic in viewer.js
ui: Show toolbar when clicking anywhere in the viewer