Fix #4540: Add hint of probable source of issue

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/Splicer.scala

@@ -140,7 +140,7 @@ object Splicer {
       try classLoader.loadClass(name.toString)
       catch {
         case _: ClassNotFoundException =>
-          val msg = s"Could not find interpreted class $name in classpath"
+          val msg = s"Could not find macro class $name in classpath $extraMsg"
           throw new StopInterpretation(msg, pos)
       }
     }
@@ -149,17 +149,19 @@ object Splicer {
       try clazz.getMethod(name.toString, paramClasses: _*)
       catch {
         case _: NoSuchMethodException =>
-          val msg = s"Could not find interpreted method ${clazz.getCanonicalName}.$name with parameters $paramClasses"
+          val msg = s"Could not find inline macro method ${clazz.getCanonicalName}.$name with parameters $paramClasses $extraMsg"
           throw new StopInterpretation(msg, pos)
       }
     }
 
+    private def extraMsg = "(the most common reason for that is that you cannot use inline macro implementations in the same compilation run that defines them)" 
+    
     private def stopIfRuntimeException[T](thunk: => T): T = {
       try thunk
       catch {
         case ex: RuntimeException =>
           val sw = new StringWriter()
-          sw.write("A runtime exception occurred while interpreting\n")
+          sw.write("A runtime exception occurred while executing macro expansion\n")
           sw.write(ex.getMessage)
           sw.write("\n")
           ex.printStackTrace(new PrintWriter(sw))