Make sure there is almost at most one *yapfify* buffer

[JorisE]

Current Situation
Whenever yapf exits with an error, a buffer is created to show the error. When a second error is caught, a new buffer is created.

Expected Situation
When a second error is caught, the existing buffer is used or the old buffers are destroyed.

[slackline]

I still see this behaviour with lots of *yapfify*<#> buffers being created.