Don't pursue SAM translation after an arity mismatch.

Before this change:

    scala> trait T { def apply(a: Int): Int }
    defined trait T

    scala> ((x: Int, y: Int) => 0): T
    <console>:9: error: object creation impossible, since method apply in trait T of type (a: Int)Int is not defined
                  ((x: Int, y: Int) => 0): T
                                    ^
After the change, these cases report the same errors as they do
*without* -Xexperimental.
Merge pull request #8 from retronym/review/3018

Don't pursue SAM translation after an arity mismatch.