Warn user when project has too many files

This is for #6962 and https://trello.com/c/fpu67Ep7/1375-brackets-8508-6962-search-issue-with-large-folder-issue.

Warn user when project has too many files, but only warn once per project session.

This nulling out is causing NPEs for me in testing -- if visit() completes synchronously (which is possible due to FS caching), we'll get here before `_getAllFilesCache()` returns, causing it to return null.

I assume Ian's intent here was that if there was a transient filesystem error, we'll just try again on the next request.  That seems a bit unlikely to fix most errors we'd see here though, so I'd be ok with just not nulling it out at all (it still gets nulled out on every FS change event, etc. so we _would_ retry again later if anything significant has changed).