SI-7544 Interpolation message for %% literal

The new error text: "conversions must follow a splice; use %% for
literal %, %n for newline".

The error is emitted when % is not at the start of a part; only
the non-conversions %% and %n are allowed.

It would be nice if it were convenient to parse the part to see
what the user may have intended, but c'est l'erreur.
In tests, interpolation is no longer -Xperimental.
StringContext#checkLengths reports bad args count

Since interpolator args are of type Any, it's easy
to write s(args) instead of s(args: _*).  I wonder
if Xlint would have warned me about that.
Merge pull request #2798 from som-snytt/issue/7544-errmsg

SI-7544 Interpolation message for %% literal