Fix invalid error responses sent from the Kotlin validator.

It was sending stderr = None essentially for every erroring case, but
stderr is (in our schema) always a string.

It never sets this anyhow, so just remove it (actually I should double
check whether *any* implementation ever sets this rather than setting
traceback or message).

Author: Julian

src/main/kotlin/Responses.kt

@@ -81,5 +81,4 @@ sealed class TestResult {
 class ErrorContext(
     val message: String? = null,
     val traceback: String? = null,
-    val stderr: String? = null,
 )