Restrict implicit args to using

Typer#adaptNoArgsImplicitMethod populates implicit args
when an arg list is missing. To remedy missing implicits,
it tries a named application `using` args it did find.
Then Applications#tryDefault supplies a default arg if
available. A previous fix to allow tryDefault to supply
implicit args for `implicit` params is now restricted
to explicit `using`; typer now adds `using` for `implicit`
when it needs to try defaults.

This commit restores propagatedFailure and the previous
condition that default params are tried if there is an error
that is not an ambiguity. An additional restriction is that
default params must be useful: there must be a param which
has a default arg to be added (because it's not a named arg).

[Cherry-picked c37dc8b][modified]

Author: tgodzik