pipeline: saner interval.

We were taking a big performance hit from updating so often, in the
long run we will go back to full smoothness but that will ask
for some profiling.
elements: remove check for too generic "AttributeError".

It was shadowing errors further down the stack.
AudioPreviewer: initialize pipeline to None, and check if it exists

before trying to stop it.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=720879