The "snap to surface" operators now have "disabled" poll messages
when there is no surface object.
The implementation in most curves operators is also unified.
The goal is to avoid having to define and use the poll failure messages
in multiple places, to reduce the boilerplate that tends to be
necessary to add an operator, and to increase the likelihood that
operators are implemented with proper poll messages.
Differential Revision: https://developer.blender.org/D15528
00a3533429