Ignore E302 "Could not rename swap file" error

According to `:help E302`, E302 comes from Vim being unable to rename
the swap file when an open buffer's name is changed; but the error is
mostly harmless. E302 seems to occur in vim-fugitive under Windows but
not *nix.

This fix allow :Glog and :Gdiff to work on such systems; previously the
uncaught error made these operations fail or at least work strangely.