R1:20ce1a905f3a

Replace AphrontUsageException with AphrontMalformedRequestException

Summary:
Ref T1806. Ref T7173. Context here is that I want to fix "you can not log in to this instance" being a confusing mess with an opaque error. To do this without hacks, I want to:

- clean up some exception handling behavior (this diff);
- modularize exception handling (next diff);
- replace confusing, over-general exceptions with tailored ones in the Phacility cluster, using the new modular stuff.

This cleans up an awkward…