cleanup ~/.maascli.db on failure

First we move up the step of setting proper ownership of ~/.maascli.db.
We also make sure to cleanup that file if failure to login occurs as
the database gets created regardless.

Fixes #199

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
Merge pull request #238 from Ubuntu-Solutions-Engineering/patch-199-fix-maasclidb-perms

cleanup ~/.maascli.db on failure [r=sparkiegeek]