ffdec: Dynamic linker errors are misreported as "file not found" errors

When reading mp3 files I get the following output:
```
Traceback (most recent call last):
2017-12-11T06:09:20.901633+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/flask/app.py", line 1982, in wsgi_app
2017-12-11T06:09:20.901634+00:00 app[web.1]: response = self.full_dispatch_request()
2017-12-11T06:09:20.901635+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/flask/app.py", line 1614, in full_dispatch_request
2017-12-11T06:09:20.901635+00:00 app[web.1]: rv = self.handle_user_exception(e)
2017-12-11T06:09:20.901640+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/flask/app.py", line 1517, in handle_user_exception
2017-12-11T06:09:20.901641+00:00 app[web.1]: reraise(exc_type, exc_value, tb)
2017-12-11T06:09:20.901641+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
2017-12-11T06:09:20.901642+00:00 app[web.1]: raise value
2017-12-11T06:09:20.901643+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/flask/app.py", line 1612, in full_dispatch_request
2017-12-11T06:09:20.901644+00:00 app[web.1]: rv = self.dispatch_request()
2017-12-11T06:09:20.901644+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/flask/app.py", line 1598, in dispatch_request
2017-12-11T06:09:20.901645+00:00 app[web.1]: return self.view_functions[rule.endpoint](**req.view_args)
2017-12-11T06:09:20.901646+00:00 app[web.1]: File "/app/flask_app.py", line 27, in display_drum
2017-12-11T06:09:20.901646+00:00 app[web.1]: song, sr = librosa.core.load(path)
2017-12-11T06:09:20.901647+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/librosa/core/audio.py", line 107, in load
2017-12-11T06:09:20.901648+00:00 app[web.1]: with audioread.audio_open(os.path.realpath(path)) as input_file:
2017-12-11T06:09:20.901648+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/audioread/__init__.py", line 111, in audio_open
2017-12-11T06:09:20.901649+00:00 app[web.1]: return ffdec.FFmpegAudioFile(path)
2017-12-11T06:09:20.901650+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/audioread/ffdec.py", line 150, in __init__
2017-12-11T06:09:20.901651+00:00 app[web.1]: self._get_info()
2017-12-11T06:09:20.901651+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/audioread/ffdec.py", line 206, in _get_info
2017-12-11T06:09:20.901652+00:00 app[web.1]: raise IOError('file not found')
2017-12-11T06:09:20.901658+00:00 app[web.1]: OSError: file not found
```

I have the following packages:
```
audioread==2.1.5
certifi==2017.11.5
chardet==3.0.4
click==6.7
cycler==0.10.0
decorator==4.1.2
Flask==0.12.2
gunicorn==19.7.1
idna==2.6
itsdangerous==0.24
Jinja2==2.10
joblib==0.11
librosa==0.5.1
llvmlite==0.21.0
MarkupSafe==1.0
matplotlib==2.1.0
numba==0.36.1
numpy==1.13.3
pyparsing==2.2.0
python-dateutil==2.6.1
pytz==2017.3
requests==2.18.4
resampy==0.2.0
scikit-learn==0.19.1
scipy==1.0.0
six==1.11.0
tabulate==0.8.2
urllib3==1.22
Werkzeug==0.13
```

ffmpeg is installed

Start in audioread/ffdec.py at _get_info(), especially the IOError raised after FFmpeg output is examined. Reproduce the MP3-loading failure with the reported FFmpeg setup and trace how dynamic-linker errors are classified. Done means linker failures are reported distinctly instead of as "file not found".

Written by the indexing model from the issue text.