title: 'collectstatic' command fails when WhiteNoise is enabled
body: <p>I'm trying to serve static files through <a href="https://github.com/evansd/whitenoise/blob/master/docs/django.rst">WhiteNoise</a> as per <a href="https://devcenter.heroku.com/articles/django-assets">Heroku</a>'s recommendation. When I run <code>collectstatic</code> in my development environment, this happens:</p>
  
  <pre><code>Post-processing 'css/iconic/open-iconic-bootstrap.css' failed!
  
  Traceback (most recent call last):
    File "./manage.py", line 10, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
      utility.execute()
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/core/management/__init__.py", line 377, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/core/management/base.py", line 288, in run_from_argv
      self.execute(*args, **options.__dict__)
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/core/management/base.py", line 338, in execute
      output = self.handle(*args, **options)
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/core/management/base.py", line 533, in handle
      return self.handle_noargs(**options)
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/contrib/staticfiles/management/commands/collectstatic.py", line 168, in handle_noargs
      collected = self.collect()
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/contrib/staticfiles/management/commands/collectstatic.py", line 120, in collect
      raise processed
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/contrib/staticfiles/storage.py", line 242, in post_process
      content = pattern.sub(converter, content)
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/contrib/staticfiles/storage.py", line 181, in converter
      hashed_url = self.url(unquote(joined_result), force=True)
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/contrib/staticfiles/storage.py", line 128, in url
      hashed_name = self.stored_name(clean_name)
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/contrib/staticfiles/storage.py", line 277, in stored_name
      cache_name = self.clean_name(self.hashed_name(name))
    File "/home/Pieter/.virtualenvs/radiant/lib/python3.4/site-packages/django/contrib/staticfiles/storage.py", line 91, in hashed_name
      (clean_name, self))
  ValueError: The file 'css/fonts/open-iconic.eot' could not be found with &lt;whitenoise.django.GzipManifestStaticFilesStorage object at 0x7f57fc5b1550&gt;.
  </code></pre>
  
  <p>The static collection command runs without incident when I comment out this line in my settings:</p>
  
  <pre><code>STATICFILES_STORAGE = 'whitenoise.django.GzipManifestStaticFilesStorage'
  </code></pre>
  
  <p>What's going wrong here and how do I fix it? I already tried emptying my static file output folder. It runs smoothly until it starts processing one specific file.</p>