Solve chicken <> egg issue.

In a plain installation, where contao is installed in the parent
directory, the plugin cannot inject the contao/core provide "early"
enough to highjack the solver. Break the installation process with an
exception, when contao is installed in the parent directory, but
composer try to install it in vendor.
Improve getContaoRoot() logic a bit.
Use custom exception for the chicken <> egg problem.