Rope reproduction steps and error popup delay.

When Rope throws an exception, Elpy will now also provide
reproduction steps similar to Jedi. Fixes #280.

Additionally, Elpy now won't pop up error descriptions for every
error, as that can be distracting from code work. Instead, if errors
happen too quickly, Elpy will simply emit a short message.

This behavior can be customized with the elpy-rpc-error-timeout option.