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/diagnosticForClassLiteral.out

@@ -1,2 +1,4 @@
-ERROR: compiler/testData/cli/js/diagnosticForClassLiteral.kt: (6, 5) Cannot translate (not supported yet): 'A::class'
+compiler/testData/cli/js/diagnosticForClassLiteral.kt:6:5: error: cannot translate (not supported yet): 'A::class'
+    A::class
+    ^
 COMPILATION_ERROR