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/diagnosticForUnhandledElements.out

@@ -1,2 +1,4 @@
-ERROR: compiler/testData/cli/js/diagnosticForUnhandledElements.kt: (9, 17) Cannot translate (not supported yet): '@fancy 1'
-COMPILATION_ERROR
+compiler/testData/cli/js/diagnosticForUnhandledElements.kt:9:17: error: cannot translate (not supported yet): '@fancy 1'
+        return (@fancy 1)
+                ^
+COMPILATION_ERROR