fix: detect demucs subprocess failure and missing output files

[octo-patch]

Fixes #68

Problem

When demucs fails during stem separation (e.g. CUDA error, missing model files, or incorrect path), it exits with a non-zero return code, but the code continued silently. The next step — ffmpeg trying to read the non-existent vocals.wav / no_vocals.wav — then crashes with a cryptic error:

Error opening input file D:\voice-pro\workspace\demucs\htdemucs\a1\vocals.wav.
Error opening input files: No such file or directory
[WinError 3] 系统找不到指定的路径: 'D:\\voice-pro\\workspace\\demucs\\htdemucs\\a1\\no_vocals.wav'

The user sees the low-level OS error rather than any indication that demucs itself failed.

Solution

• After the Popen context exits, check sp.returncode. If it is non-zero, raise a RuntimeError with a clear message pointing the user to the console output.
• Add an os.path.exists guard on both expected output files before calling ffmpeg. If either file is missing (even when demucs returns 0), raise a FileNotFoundError with the output directory path so the user knows where to look.

Testing

Reviewed the demucs_split_file function and traced the error reported in the issue to the missing return-code check. The fix is minimal and does not change any logic paths when demucs succeeds.