fix: improve missing FFmpeg error for ASR (#3478)

* Improve missing FFmpeg error for ASR

Signed-off-by: Simon Dräger <sfdraeger@gmail.com>

* fix: simplify missing ffmpeg handling for ASR

Signed-off-by: Simon Dräger <sfdraeger@gmail.com>

* test(asr): move commonly-used imports to top

Move common lightweight imports at the top while
keeping expensive model class imports localized
(lazy loading) for optimal test performance.

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

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Signed-off-by: Simon Dräger <sfdraeger@gmail.com>
Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>
Co-authored-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>