Raise error when file does not contain a cask.

Library/Homebrew/cask/lib/hbc/cask.rb

@@ -17,7 +17,11 @@ module Hbc
       return to_enum unless block_given?
 
       Tap.flat_map(&:cask_files).each do |f|
-        yield CaskLoader::FromTapPathLoader.new(f).load
+        begin
+          yield CaskLoader::FromTapPathLoader.new(f).load
+        rescue CaskUnreadableError => e
+          opoo e.message
+        end
       end
     end
 

Library/Homebrew/cask/lib/hbc/cask_loader.rb

@@ -54,7 +54,15 @@ module Hbc
 
         @content = IO.read(path)
 
-        instance_eval(content, path)
+        begin
+          instance_eval(content, path).tap do |cask|
+            unless cask.is_a?(Cask)
+              raise CaskUnreadableError.new(token, "'#{path}' does not contain a cask.")
+            end
+          end
+        rescue NameError, ArgumentError, ScriptError => e
+          raise CaskUnreadableError.new(token, e.message)
+        end
       end
 
       private

Library/Homebrew/cask/lib/hbc/exceptions.rb

@@ -36,6 +36,12 @@ module Hbc
     end
   end
 
+  class CaskUnreadableError < CaskUnavailableError
+    def to_s
+      "Cask '#{token}' is unreadable" << (reason.empty? ? "." : ": #{reason}")
+    end
+  end
+
   class CaskAlreadyCreatedError < AbstractCaskErrorWithToken
     def to_s
       %Q(Cask '#{token}' already exists. Run #{Formatter.identifier("brew cask edit #{token}")} to edit it.)