Simplify-pass follow-up on the #68157 salvage (regression-neutral \u2014 the
old temp-file version was equally bare): capture ffmpeg's -loglevel
error output so a CalledProcessError carries the real message, and log
it at the voice-input catch site. Parity with transcription_tools'
ffmpeg call sites. Live-verified: forced ffmpeg failure produces the
captured message in the exception.