After ‘C-x v g’ (vc-annotate) typing ‘a’ (vc-annotate-revision-previous-to-line)
in the *Annotate* buffer on the earliest (initial) revision fails with:

  Debugger entered--Lisp error: (error "Invalid argument to vc-annotate-warp-revision")
    signal(error ("Invalid argument to vc-annotate-warp-revision"))
    error("Invalid argument to vc-annotate-warp-revision")
    vc-annotate-warp-revision(nil "...")

This patch provides a nicer message: