Remove extra % from ffmpeg encoding command.

I should really just delete the command, since it's entirely
not-frame-accurate and therefore wholly worthless.
Make windowerr cancel the macro instead of throwing an error.

The purpose of windowerr is to present the error message more obviously
in its own window. Thus, after the error message has been closed, the
user should not have to additionally close the automation log window.
Spawn a proper error when 'Video File' header is not found.

Hopefully this covers the `[string "Aegisub-Motion.lua"]:1283: attempt
to index a nil value` reports I've been getting. This bug seems to be
NotMyFault, as Aegisub delays writing/modifying the Video File header
until the file is saved in order to avoid setting an undo point that
could cause problems.
Change the way encode.bat is deleted.

Apparently for some people `%0` was leaving out the .bat extension.
This should fix that problem, hopefully.