Exception when persisting class unlisted in disciminator map

[jmikola]

For context: #788 (comment)

Behavior for 1.0 is to write the FQCN in the discriminator field if the particular class is not listed in the map (#865). This could lead to situations where the FQCN in the database is no longer relevant (e.g. classes were moved or renamed in code). That situation will likely always be possible when target document mappings are omitted, but if the user is opting into use a discriminator map, we can assume that they want all classes to be in that map. Throwing an exception would be an extra precaution.