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/integration/smoke/syntaxErrors/test.compile.expected

@@ -1,5 +1,7 @@
 
 ERR:
-ERROR: test.kt: (4, 20) Expecting an element
+test.kt:4:20: error: expecting an element
+    val s = System.in
+                   ^
 
 Return code: 1