Fix issues evolving an extension's models.
We weren't properly calling Django Evolution's evolve management

command. We needed to go through execute() and not evolve(), in order to

get objects like stdout and stderr set up. Without this, we get an

exception when Django Evolution tries to output any text.
While fixing this, I changed it to take the output from the evolve

command and include it in the stored error. It's now shown to the user,

and logged, when it fails. It also logs when it succeeds and there's

something to show.

Simulated a failed evolution. Saw the error in the log and in the displayed

error output.
Simulated a successful evolution. Saw the log information.