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/wrongScriptWithNoSource.out

@@ -1,2 +1,2 @@
-ERROR: Specify script source path to evaluate
-COMPILATION_ERROR
+error: specify script source path to evaluate
+COMPILATION_ERROR