Rope backend now uses a central call interface.

Most calls to Rope follow similar call signatures and need to catch
the same exceptions. Factor out all of this into a common function
similar to the Jedi backend.
Show the current flymake error in the echo area.

When the current line is highlighted by flymake, Elpy will now
display the flymake error for that line instead of the function
call signature.

Fixes #232.