[Downloader] ensure that a downloader failure results in a DownloaderError so…

[Downloader] ensure that a downloader failure results in a DownloaderError so the Downloader can catch it

Helpful for https://github.com/CocoaPods/cocoapods-downloader/pull/36

lib/cocoapods/downloader.rb

 require 'cocoapods-downloader'
+require 'claide/informative_error'
 
 module Pod
   module Downloader
+    class DownloaderError; include CLAide::InformativeError; end
+
     class Base
       override_api do
 
         def execute_command(executable, command, raise_on_failure = false)
           Executable.execute_command(executable, command, raise_on_failure)
+        rescue CLAide::InformativeError => e
+          raise DownloaderError, e.message
         end
 
         # Indicates that an action will be performed. The action is passed as a