title: How to raise error in a rails app?
body: <p>I'm trying to upload a file using gem 'roo'. When I check for file of type, I have this in a instance method:</p>
  
  <pre><code>def open_spreadsheet
    case File.extname(file.original_filename)
      when ".xls" then Roo::Excel.new(file.path, file_warning: :ignore)
      when ".xlsx" then Roo::Excelx.new(file.path, file_warning: :ignore)
      when ".csv" then Roo::CSV.new(file.path, file_warning: :ignore)
      when ".ods" then Roo::LibreOffice.new(file.path,file_warning: :ignore)
      else raise "Unknown file type"
    end
  end
  </code></pre>
  
  <p>Is there a way to catch this exception so that user sees just message and gets to try again, without actually raising syntax error?</p>