if dlload fail to load a library, it should raise an error.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@8594 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/dl/lib/dl/import.rb

@@ -49,7 +49,7 @@ module DL
           begin
             DL.dlopen(lib)
           rescue DLError
-            nil
+            raise(DLError, "can't load #{lib}")
           end
         end
       }.flatten()