Replace Critical Asserts

[Vollkornaffe]

We shouldn't rely on assertions for logic-critical things; they can be stripped in optimized settings.

Instead, we should use branching with dedicated Exception classes.

Related: https://discuss.python.org/t/stop-ignoring-asserts-when-running-in-optimized-mode/13132

[Vollkornaffe]

Blender probably never runs Python with -O, so this may only be important in other tooling around Tree Clipper?