Fail with a helpful error if :head is unsupported.

lib/cocoapods/installer/pod_source_installer.rb

@@ -95,11 +95,17 @@ module Pod
       def download_source
         root.rmtree if root.exist?
         if head_pod?
+          begin
             downloader.download_head
-          if downloader.options_specific?
             @specific_source = downloader.checkout_options
+          rescue RuntimeError => e
+            if e.message == 'Abstract method'
+              raise Informative, "The pod '" + root_spec.name + "' does not " + 
+                "support the :head option, as it uses a " + downloader.name + 
+                " source. Remove that option to use this pod."
             else
-            @specific_source = {}
+              raise
+            end
           end
         else
           downloader.download