Better compile-time diagnostics when attempting to extract case classes from JSON

diagnostics.repl

scala> case class Bar(file: java.io.File) // We can't extract a File
defined class Bar

scala> case class Foo(bar: Bar) // therefore we can't extract a Bar
defined class Foo

scala> json""{ "bar": { "file": "..." } }""".as[Foo]
Could not generate a Json extractor for case class Bar because a Json extractor for parameter `file' of type java.io.File could not be found
Could not generate a Json extractor for case class Foo because a Json extractor for parameter `bar' of type Bar could not be found
<console>:21: error: cannot extract type Foo from rapture.json.Json.
       json"""{ "bar": { "file": "..." } }""".as[Foo]
                                                ^