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/compilationFailed/hello.compile.expected

@@ -1,5 +1,7 @@
 
 ERR:
-ERROR: hello.kt: (4, 5) Unresolved reference: a
+hello.kt:4:5: error: unresolved reference: a
+    a
+    ^
 
 Return code: 1