cf42c511d8 primitives: replace error propagation with `expect` (Andrew Poelstra)
Pull request description:
This error path cannot happen, and if it could, failing formatting is not the correct source of action, because the std docs say that formatting may only fail if the formatter says so.
Fixes comment in #4269
ACKs for top commit:
tcharding:
ACK cf42c511d8
Kixunil:
ACK cf42c511d8
Tree-SHA512: 0ad47bb210dc71568bde426b8fd7782a4d48415fbf27133783975117d174b58eb791e68290e53f150a7e65236efda75032852bb6ed30bf4a182f18b9440cfcf9