Fixes #413, whereby the NamedTemporaryFile "dest" was out of scope in the error handling portion of rewrite.

[theGOTOguy]

I just changed the with block to dest = ... since delete=False was preventing it from being cleaned up anyway, and then set a default value for dest in case an error happens earlier.

[theskumar]

Thanks, @theGOTOguy, for the fix!