Improve error messages when the JDBC connector isn't properly set up

When setting up a Tomcat instance without properly reading the install instructions and especially without putting the correct JDBC driver in the WEB-INF/lib folder, we end up with the attached stacktrace.

This very stacktrace is quite missleading as the user might think that he made an error in the configuration files at first, when the error actually comes from a JAR not being present. We could add a small check and display a more user-friendly message for that kind of problem.