1) The error comes from MapPress because it issues a database command to check for a table (see #2). The error is just a symptom of the real problem – which is that the server lost its connection to the database – it’s not an indication that the error has anything to do with MapPress or that specific table.
2) WordPress loads every plugin for every page. When MapPress is loaded it checks that its table exists, whether or not it’s displaying a map. That’s why you see an error on pages without a map.