JS DCE: drop unknown file report severity to WARNING

*.kjsm and other files might be received when FileCollection is
used in Gradle as a dependency.

Example: `testCompile project(":$coroutines_core").sourceSets.test.output`
(a popular-ish solution to introduce dependencies between tests)

compiler/tests/org/jetbrains/kotlin/cli/CliTestGenerated.java

@@ -783,6 +783,12 @@ public class CliTestGenerated extends AbstractCliTest {
             doJsDceTest(fileName);
         }
 
+        @TestMetadata("invalidFilename.args")
+        public void testInvalidFilename() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/cli/js-dce/invalidFilename.args");
+            doJsDceTest(fileName);
+        }
+
         @TestMetadata("jsExtraHelp.args")
         public void testJsExtraHelp() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/cli/js-dce/jsExtraHelp.args");