In MDL-67822 we exposed the schema cli as a check. but its is fairly simplistic in that any form of misalignment is considered an error.

For example if a whole table is missing this should be a critical error. If a table is missing a column this should be an error. If a column has the wrong default value that should also be an error. 

But if there is an extra table present, eg a plugin has been uninstalled partially which is bad but should be a warning rather than an error and there is no loss of function here.

If there is an extra index then this could be an 'info' as it is fairly common for extra indexes to be added from time to time to solve specific performance issues, and at some later time these end up contributed back into core or into a plugin once they have proven themselves.