Rescue ::SyntaxError when checking Ruby code

Since Haml::Exec::Haml is inside the top Haml module, "SyntaxError"
refers to "Haml::SyntaxError", not the top level "SyntaxError".

Use "::SyntaxError" to make sure we rescue the right one, and avoid
the need to rescue "Exception".

lib/haml/exec.rb

@@ -336,8 +336,7 @@ def process_result
       def validate_ruby(code)
         begin
           eval("BEGIN {return nil}; #{code}")
-        # Not sure why, but rescuing "SyntaxError" does not work here.
-        rescue Exception
+        rescue ::SyntaxError # Not to be confused with Haml::SyntaxError
           $!
         end
       end