Fix error if god-execute-with-current-bindings deletes buffer

[Drainful]

Currently if a command executed via god-execute-with-current-bindings results in the deletion of the current buffer, an error message is spat out in the minibuffer. This commit updates the cleanup process of god-execute-with-current-bindings to check if the buffer exists and is alive before disabling god-local-mode.

Fixes #117

[darth10]

Thanks for reporting and fixing this bug @Drainful!
Would it be possible for you to add a test for this fix?

[Drainful]

I'm not sure how to write a test since the error isn't caught by debug-on-error. It seems like the only artifact of the error is the annoying message in the minibuffer as it goes unhandled. I'm not sure why this is the case; it may have something to do with the error being raised during the function passed as the on-exit parameter of set-transient-map which ultimately ends up added to pre-command-hook.

[darth10]

This might be easier to test with ert than with feature-based testing through ecukes.
For now, I'll go ahead and merge it 👍

[darth10]

Thanks for the contribution @Drainful! 💯

[Drainful]

NP! thanks for your work on this awesome package. I feel like god-mode has filled a massive hole in my emacs config.