fix and validate core selection

toolflow/src/main/scala/com/minres/tgc/hammer/cli/CoreSelection.scala

@@ -5,6 +5,13 @@ import org.rogach.scallop.*
 
 trait CoreSelection { this: MySubcommand =>
   val core: ScallopOption[String] = opt[String](group = mainGroup, required = true, descr = "The core to be extended; core datasheets are in coreDatasheets/")
+  def getCoreDatasheet: os.Path = CORE_DATASHEETS / s"${core()}.yaml"
 
-  def getCoreDatasheet: os.Path = CORE_DATASHEETS / s"$core.yaml"
+  addValidation {
+    if (os.isFile(getCoreDatasheet)) {
+      Right(())
+    } else {
+      Left(s"Core ${core()} not supported, no core datasheet at ${getCoreDatasheet}")
+    }
+  }
 }

toolflow/src/main/scala/com/minres/tgc/hammer/tasks/Task.scala

@@ -19,6 +19,8 @@ trait TaskImpl[T <: Task : ClassTag] extends Task with Logging[T] {
 
   def runExecutable(execPath: Path, args: Shellable*): os.CommandResult = {
     val command = s"$execPath ${args.flatMap(_.value).mkString(" ")}"
+    log.info(s"Running external program: ")
+    log.info(command)
     os.proc("bash", "-c", command).call(stdout = os.Inherit)
   }
 }