Catch NoBackendError exception and tell user to install ffmpeg

[ghost]

Despite instructions to install ffmpeg in README.md (#414), people still skip it and get audioread's NoBackendError whenever trying to load mp3s. The exception is confusing to the end user and we get a lot of questions about it.

• The error message looks like this: Toolbox can't vocode. #386 (comment)
• What people need to do to resolve it is this: Common toolbox issues and how to fix them #431 (comment)

Here are some options for dealing with it, sorted in ascending order of preference.

Option 1: Catch it when it occurs

This would make a nice starter issue for someone who is just getting started with python:

In demo_cli.py and demo_toolbox.py,

1. Use try/except around librosa.load()
   • Lines 132-138 of demo_cli.py
   • Lines 153-156 of toolbox/__init__.py
2. Catch the NoBackendError exception (this will require you to import NoBackendError from audioread.exceptions)
3. Print a helpful error message instructing the user to install ffmpeg for mp3 support.

Option 2: Test for mp3 support as toolbox is loaded

This is a little more advanced, but I think this is preferred.

1. As the toolbox is opened, or when demo_cli.py is started, we either use librosa.load() or audioread.audio_open() on a sample mp3 file to test the capability.
2. If we catch NoBackendError we nag the user and tell them to either install ffmpeg, or rerun the toolbox with --no_mp3_support
3. The --no_mp3_support flag disallows loading of mp3s when one is selected, preventing NoBackendError from being encountered.

Option 3: Check for ffmpeg backend as toolbox is loaded

Although other backends are able to open mp3, we can force a check for Windows users to see if they followed the instructions and installed ffmpeg. Then do the same nag as option 2 if not detected.

audioread.ffdec.available() can test for ffmpeg availability: audioread/ffdec.py

Take a look at audioread/__init__.py and see how it raises NoBackendError. The code is extremely simple.

Option 4: Contribute to audioread to make the error message more descriptive

beetbox/audioread#104

[ghost]

This is resolved by #517. Thank you for the contribution @ramalamadingdong .

[ramalamadingdong]

Thanks for the support!