Upon failure, don't destroy the underlying director implementation. When
the deletion code was split in c671bb66
in order to be reused in 61529854, it
mistakenly included the director implementation.

Instead, it's really the caller's job to tear down the backend if it
couldn't be added. Since vbe_destroy operates on a director, it is split
in two to avoid reintroducing the previous "undo" code duplication.

Refs #3176