CLI: improve diagnostic message format

- render the whole line where the error/warning points to, if any, and another
  line with '^', like other compilers do
- lowercase diagnostic severity
- decapitalize the message if it doesn't start with a proper name

compiler/testData/cli/jvm/duplicateSourcesInModule.out

@@ -1,2 +1,2 @@
-WARNING: Duplicate source root: $TESTDATA_DIR$/duplicateSourcesInModule.kt
-OK
+warning: duplicate source root: $TESTDATA_DIR$/duplicateSourcesInModule.kt
+OK