Remove superfluous library includes.

I don't need these.
Model should not format result.

This is considered a presentation on concern.  Just return the useful
object.
Throw a more useful error

die with "unable to connect" isn't very useful.
Remove a redundant else
Remove another redundant else and connection close

Don't need else because exception (now throwing a useful error) will
stop the script.  Also removed $mysqli->close();