Fail zip code population conditional when zip code is not found in the database

[jzonthemtn]

If a zip code is not found in the local database, its population will be returned as -1.

Conditional checks involving zip code population should not consider -1 as a valid population. For instance, the zip code 12345 will return -1, and a condition that says "population < 500" will be evaluated as true. We can't be certain that is actually a true statement so the conditional should fail.

See #173 for a little more context.