describe-variable with string arg

[Drew Adams]

describe-variable expects a symbol, not a symbol-name (string).

In Emacs 20, passing it a symbol name leads to an error message. Starting
with Emacs 21, the message is displayed via `message', not `error'. AFAICT,
no Emacs code depends on this use of `message' instead of `error'. The
latter makes more sense to me.

Beyond that, why not let `describe-variable' try a little harder, to use
`intern' applied to its arg, if that is a string?

That would be friendlier to possible 3rd-party code that might generically
apply a function such as `describe-variable' to an arg that might be a
symbol name instead of a symbol.