Fix trascriber error handling and support mp4 conversion

[Latemus]

• Fixed TranscribeCommand error handling so that it now informs if there was an error. Problem: TranscribingError did not call super in init and as such raised error itself when it was caught
• Fixed TranscribeCommand media conversion so that it supports mp4 files. Problem: Conversion from mp4 to mp3 exported the audio in some codec that the OpenaiAPI did not support. Now all audio is converted to mp4 without video (m4a) and then sent to OpenaiAPI
• Added information to readme, that ffmpeg-program is required to run the bot. Added warning to the startup that informs, if ffmpeg is not available. Added test that fails if ffmpeg is not available.
• Removed italics styling from transcribe result message

[Latemus]

@taturl Testikattavuus lisätty. Valmis uudelleenarviointia varten. Samalla poistettu ffmpeg riippuvuus paikallisesta ajamisesta ja lisätty tiedot reamde.md, että se on riippuvuus jonka tarvitsee vain muutamaan komennon paikalliseen ajamiseen

[Latemus]

@taturl Laitettu automerge päälle, joten jos korjattavaa niin suoraan vain requires changes

[Latemus]

Nappasin nyt automergen kuitenkin pois, jos tulee jotain pientä korjattavaa kuten kirjotusvirhettä tms, niin voi antaa approvalin ja korjattavan kohdan tms.

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