Show a friendly error if you use the validate option but forget: %s

before:
````
fatal: [192.168.33.10] => failed to parse: Traceback (most recent call last):
  File "/home/vagrant/.ansible/tmp/ansible-tmp-1397870557.51-203219450016801/copy", line 1326, in <module>
    main()
  File "/home/vagrant/.ansible/tmp/ansible-tmp-1397870557.51-203219450016801/copy", line 210, in main
    (rc,out,err) = module.run_command(validate % src)
TypeError: not all arguments converted during string formatting
````

after:
````
failed: [192.168.33.10] => {"failed": true, "item": "", "md5sum": "cb9915cece456389db174d2e69311f57"}
msg: validate must contain %s: /bin/true
````