Added hostIdentifier to info output

Small change that adds the the hostIdentifier in the output to the info log when running queries.

Also, wraps the `DBHandle::getInstance()` call in a try block, an cause the `getHostIdentifier()` function to throw an error if it cant access the DB.
I thought it made a little more sense now that `DBHandle::getInstance()` is wrapped in a try block. If there are any db-related errors while trying to get the hostIdentifier, and error is thrown. The logic for handling that error (in this case, defaulting to the hostname) is all in one place.

I can change this to return an osquery::status, if that is preferred. Either way works.