Provide better error when main class is not an object

[keynmol]

//> using platform scala-native

class App:
  def main(args: Array[String]): Unit = println("hello")

Results in a scary looking error:

> scli run . -M App
Compiling project (Scala 3.3.1, Scala Native 0.4.17)
Compiled project (Scala 3.3.1, Scala Native 0.4.17)
[error] Found 1 missing definitions while linking
[error] Not found Member(Top(App),D4mainLAL16java.lang.String_uEo)
Exception in thread "main" scala.scalanative.linker.LinkingException: Undefined definitions found in reachability phase
        at scala.scalanative.linker.Reach.fail(Reach.scala:979)
        at scala.scalanative.linker.Reach.reportMissing(Reach.scala:974)
        at scala.scalanative.linker.Reach.result(Reach.scala:52)

On 0.5.0 is somewhat better, but still:

[info] Discovered 871 classes and 5425 methods after classloading
[error] Found 1 unreachable symbols!
[error] Unknown static method App.main(java.lang.String[]): scala.runtime.BoxedUnit, referenced from:


Exception in thread "main" [info] Total (1106 ms)
scala.scalanative.linker.LinkingException: Unreachable symbols found after classloading run. It can happen when using dependencies not cross-compiled for Scala Native or not yet ported JDK definitions.
        at scala.scalanative.build.ScalaNative$.$anonfun$withReachabilityPostprocessing$3(ScalaNative.scala:94)
        at scala.concurrent.Future.$anonfun$flatMap$1(Future.scala:307)