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/js/libraryDirNotFound.out

@@ -1,2 +1,2 @@
-ERROR: Path 'not/existing/path' does not exist
+error: path 'not/existing/path' does not exist
 COMPILATION_ERROR