throbber: don't report successful load when no assets

Currently, the throbber will report a successful load when zero assets
are loaded. Change it so that zero assets do not result in a
successfully loaded throbber.
throbber: don't report success when no frames were added

Currently, the throbber will report success when assets are *found* in
the directory. When loading those assets, code will apply more checks to
determine if the assets are meant for the throbber.
In case none of them are, we'll end up not adding any frames, so we
should not report a successful load.
two-step: end_animation can be NULL

After the past few commits, view->end_animation can be NULL when no
asset is found for it.
When that is the case, do not try to render an end animation.
Merge pull request #4 from endlessm/shell/3769

Shell/3769