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/nonExistingSourcePath.out

@@ -1,2 +1,2 @@
-ERROR: Source file or directory not found: not/existing/path
+error: source file or directory not found: not/existing/path
 COMPILATION_ERROR