title: Setting Custom error message with CarrierWave
body: <p>I have an Image uploading application running on carrierwave  which  restrict user from uploading image only of desired extensions 'jpg,jpeg,png' I have put in place the validation for carrierwave in my Uploader define as</p>
  
  <pre><code>def extension_white_list
    %w(jpg jpeg png)
  end
  </code></pre>
  
  <p>Now anyone trying to upload image having extension different from the desired extension(mention above) will result in validation errors </p>
  
  <p>I want to customize the Validation Error message </p>
  
  <p>Right Now Error message is presented as </p>
  
  <pre><code>You are not allowed to upload "" files, allowed types: ["jpg","jpeg","png"]
  </code></pre>
  
  <p>Can anyone help I found this <a href="https://github.com/bartt/carrierwave/commit/5585a1fd5a8207be653da9ba4236369a5db66428">link</a> where the  guys mention how to achieve this</p>
  
  <p>but it some problem</p>
  
  <pre><code>1. I18n support I dont require to translate the error message using I18n 
  
  2. The 'Key' to be used I not sure which key to used in YAML for
      not matching extension whitelist error message (e.g) carrierwave_processing_error key if error is for processing failure
  </code></pre>
  
  <p>Please Provide me answer in term of CarrierWave and please dont ask me to write separate validation set for extension matching </p>