Returns 500 instead of 404

[vizath]

Hi,
Thanks for that middleware.

We moved from 0.0.4 to 0.1.4 this week and we started getting 500 on touch icons.
Turns out that errno is not always 34 on missing filenames:
https://github.com/theworkers/connect-favicons/blob/master/lib/favicons-middleware.js#L63

eg, on Windows

{ [Error: ENOENT: no such file or directory, open 'C:\users\...\public\images\icons\apple-touch-icon-precomposed.png']
  errno: -4058,
  code: 'ENOENT',
  syscall: 'open',
  path: 'C:\\users\\...\\public\\images\\icons\\apple-touch-icon-precomposed.png' }
Error: ENOENT: no such file or directory, open 'C:\users\...\public\images\icons\apple-touch-icon-precomposed.png'
    at Error (native)

We ended up adding the missing -precomposed files.