EventIndexPanel: Catch getStats failures.
Getting the stats can fail when used with Seshat. Tantivy periodically
garbage collects its files. Smaller files are merged and the old ones
are removed.
If garbage collection occurs while we try to get the stats, which go
trough the files and figure out their sizes, we can end up trying to
figure out the file size of a removed file. The getStats call will fail
in this case but we can ignore the failure since we will likely get a
nice result next time we try.
