Both vc-dir-mode and dired-mode are supported in vc-deduce-fileset,
but only vc-dir-mode is supported in vc-ensure-vc-buffer.

This patch prepares a VC-controlled file buffer from Dired as well.

Error-checking is moved outside of the last branch of cond,
because in addition to dired-mode, it's also needed for vc-dir-mode
to avoid the failure with a backtrace when a single-file operation
is executed in vc-dir-mode on a directory.

Also I noticed that the check ‘(unless backend (error "Directory not under VC"))’
is not needed anymore because now vc-responsible-backend raises the same error.
There are more such useless checks in vc.el that could be removed as well.