Presently, WiredTigerKVEngine::alterIdentMetadata uses invariantWTOK on its call to WiredTiger alter. As a result, if the alter fails for some reason, e.g., "No such file or directory", it results in the following cryptic log line:

Invariant failure: session.getSession()->alter(session.getSession(), uri.c_str(), alterString.c_str()) resulted in status UnknownError: 2: No such file or directory at src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp 1341

It would be more helpful to specify extra information about the offending index, e.g., the URI string, if the alter call fails.