js2c: raise proper Exception, not a string

Fixes the following error message:

  TypeError: exceptions must be old-style classes or derived
  from BaseException, not str

Fixes #4303.

tools/js2c.py

@@ -206,7 +206,7 @@ def ReadMacros(lines):
           fun = eval("lambda " + ",".join(args) + ': ' + body)
           macros[name] = PythonMacro(args, fun)
         else:
-          raise ("Illegal line: " + line)
+          raise Exception("Illegal line: " + line)
   return (constants, macros)