Parsing invalid DDI should raise MappingError (citation with no titlStmt)

[Toni Sissala]

If a DDI2.5. has no titlStmt inside relPubl/citation, the parser raises an AttributeError. It should raise MappingError instead. This makes it more explicit and reliable to catch in caller.

[Toni Sissala]

Fixed in 4cd20b4c58731b96fe085b3ccb6cae9b62f8596d