Avoid double errors when tasty file does not exist

There is no point trying to load the tasty file after we have emitted the error.
Loading it only end up in a second error that shadows the root of the problem.

Also improve readabllility of the error message.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/Driver.scala

@@ -89,23 +89,23 @@ class Driver {
       // Resolve classpath and class names of tasty files
       val (classPaths, classNames) = fileNames0.flatMap { name =>
         val path = Paths.get(name)
-        if (name.endsWith(".jar"))
+        if name.endsWith(".jar") then
           new dotty.tools.io.Jar(File(name)).toList.collect {
             case e if e.getName.endsWith(".tasty") =>
               (name, e.getName.stripSuffix(".tasty").replace("/", "."))
           }
         else if (!name.endsWith(".tasty"))
           ("", name) :: Nil
-        else if (Files.exists(path))
+        else if Files.exists(path) then
           TastyFileUtil.getClassName(path) match {
             case Some(res) => res:: Nil
             case _ =>
-              report.error(s"Could not load classname from $name.")
-              ("", name) :: Nil
+              report.error(s"Could not load classname from: $name")
+              Nil
           }
         else {
-          report.error(s"File $name does not exist.")
-          ("", name) :: Nil
+          report.error(s"File does not exist: $name")
+          Nil
         }
       }.unzip
       val ctx1 = ctx0.fresh