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 "AphrontUsageException" which does some weird stuff right now. In particular, it is extensible and extended in one place in Diffusion, but that extension is meaningless.

Realign this as "AphrontMalformedRequestException", which is a better description of what it is and does: raises errors before we can get as far as normal routing and site handling.

Hit some of these exceptions, saw the expected "abandon all hope" error page.