Provide helpful exception for non-PATH-like conflict with admin.user.env

Environment variables specified in mapreduce.admin.user.env are supposed to be paths (class, shell, library) and they can be merged with the user-provided values. However, it's also possible that the cluster admins specify some non-PATH-like variable such as JAVA_HOME. In this case if there is the same variable provided by the user, we'll get a concatenation that does not make sense and is difficult to debug.