Unhelpful error message when there are too many Tag options

Summary: This becomes obvious, now that I am dropping the minimum number of characters for Manifest prompts from 3 to 1. This means that we frequently get too many options back, which blows the Manifest maximum. The resulting error message is "Your search could not be completed at this time". It should be "Many possible completions -- please type more", or something like that.

Ideally, the excessive number should be detected on the server end, and it should send a signal back that tells Manifest to show the message, to spare us useless wire traffic.