routes/registration: Simplify error handling for creates/updates
routes/registration: Simplify error handling for creates/updates

There's now only 1 error handler, which returns a 500 error and logs the
message internally.

Previously failed creates would do this, while failed updates would send
the form again with a 'save_error' property set. No templates actually
use 'save_error' so this looks vestigial.

thanostx/regcfp,thanostx/regcfp,thanostx/regcfp