To be able to use it correctly inside an xml build file where, if the
path is not absolute, it's treated as relative to the build file path
(which is TMP_DIR in several tests of the subsequent commit)
- 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
Alexander Udalov