Validation of LMF IDs when adding lexicons

SQL errors can be hard to understand, especially for a user of Wn. To avoid these, some validation of the LMF files should be performed, such as ensuring that all IDs referenced are provided by the document (including as external elements in extension lexicons).

There is already some validation, e.g., of allowed part of speech values, relation types, etc, but so far nothing regarding entity linking. Things like cycle detection are probably too expensive to do during add, though.

Start by locating the existing validation performed when LMF files are added, then inspect the tests covering allowed values and relation types. Define the validation boundary for entity links, including IDs supplied as external elements in extension lexicons; done means invalid references are rejected before they produce SQL errors without adding cycle detection.

Written by the indexing model from the issue text.