fix imagefreeze ci failure seeking while/before negotiating.

commit b3d5ad74

imagefreeze: handle setcaps failures

Previously we would ignore all failures configuring caps and would
continue attempting to calculate times without a framerate.

Now we signal the negotiation failure with an element message or
pause the srcpad task on flushing.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/issues/705

commit 5344fe27

imagefreeze: don't fail sending sticky events downstream

They will be repropagated anyway.