Introduce a base exception class

[emyller]

This introduces the PydubException base exception class.

It's a very small contribution but makes Pydub-related errors more consistent and a bit easier to handle:

@celery_app.task(bind=True, max_retries=3)
def process_audio(self, audio_src: str) -> bytes:
    """
    Find and convert a WAV audio source into MP3
    """
    try:
        audio_bytes = retrieve_audio_file(audio_src)
        wav_audio = pydub.AudioSegment.from_wav(wav_container)
        mp3_container = io.BytesIO()
        wav_audio.export(
            mp3_container,
            codec='libmp3lame', format='mp3', parameters=['-q:a', '5'],
        )
    
    # The file might be taking a bit longer to show up
    # Or ffmpeg is being flaky (it can happen for mysterious reasons)
    except (FileNotFoundError, PydubException,) as err:
        self.retry(exc=err)
    
    else:
        return mp3.read()

...And also fix some PEP8 complaints.