check for cask before unlinking during uninstall

this was exposed in #313 by @vitorgalvao in his attempt to update the
anvil cask; by adding a link to that cask, (which just so happens to be
the cask that we chose for our "already uninstalled message" test).

the fix is simply to move the "already uninstalled" check *before* we
start to unlink.

Casks/anvil.rb

@@ -1,6 +1,7 @@
 class Anvil < Cask
-  url 'http://anvilformac.com/download'
+  url 'http://sparkler.herokuapp.com/apps/3/download'
   homepage 'http://anvilformac.com/'
   version 'latest'
   no_checksum
+  link :app, 'Anvil.app'
 end

lib/cask/cli/uninstall.rb

@@ -3,10 +3,11 @@ class Cask::CLI::Uninstall
     begin
       casks = cask_names.map { |cn| Cask.load(cn) }
       casks.each do |cask|
+        raise CaskNotInstalledError.new(cask) unless cask.installed?
         Cask::AppLinker.new(cask).unlink
         Cask::Installer.uninstall(cask)
       end
-    rescue CaskUnavailableError,CaskNotInstalledError => e
+    rescue CaskError => e
       onoe e
     end
   end

lib/cask/installer.rb

@@ -28,8 +28,6 @@ class Cask::Installer
     end
 
     def uninstall(cask)
-      raise CaskNotInstalledError.new(cask) unless cask.installed?
-
       cask.destination_path.rmtree
     end