treat IOException in index check as success

[vladak]

As suggested in #4401 (reply in thread), the index check should treat IOException separately. If such error happens, the indexer exits with return code of 2. The exit code of 1 is restricted to legitimate index check errors. The Docker startup script will wipe out the index (with CHECK_INDEX set) only if indexer program returns 1.

This also changes how the checks are done by the indexer. Previously, the index checks were cumulative, such that when duplicate document check was specified, the indexer also performed version check. This is no longer true, the check modes are now exclusive.

Also, new check for definitions was added, however at this stage this is experimental as there are variances and nuances how definitions are generated for various files. The check takes the definitions as produced by ctags and loosely checks if they are present in the corresponding source file.

For the test of IOException being throw, I tried using static method mocking (had to update to Mockito v5 first), however had no luck with that, so the test is based on IOException being thrown for non-existent directory. Should probably bite the bullet and convert the IndexCheck class to non-static/utility so that normal mocking can be done.