Assorted screensaver.js bug fixes.

[Gollimnar]

As mentioned in #171 -

Setting video.src = "" or video.src = null causes an error event, at least in the edge engine (chromium?). Setting it to video.removeAttribute( "src" ) has the correct result.

The error events in "pre-player" would fail to log the reason for the error properly.

prepVideo() didn't handle failure to find the video by ID properly - the workaround just logs this and lets it skip to another video instead of a dereferencing a null value.

The fade/down and wipe/down transitions should have read 'bottom' instead to match the transition code. They triggered a null dereference exception as a result.

Any errors inside drawVideo() - like transitions - result in drawVideo() not being queued up again for the next animation frame, freezing the screensaver.

The 'blankTheScreen' message repeats and would cause a slow strobe effect as the fadeout effect would trigger each time.

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Note that two outstanding screensaver bugs still seem to exist and aren't fixed by this. In each case they were only observed once and I couldn't duplicate them:

• An unknown error that was probably caused by the prepVideo() issue.
• An error during fadeout that causes it to terminate with the video showing instead of a blank screen at the end of the effect.

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Leaving the screensaver running for extended periods, I can no longer trigger any further bugs.