Remove custom type errors from Member

[isovector]

This PR removes custom type errors from usages of Member. Now that polysemy-plugin is stable, most of the catchable, useful errors are now magically solved. What remain are mostly over-eager errors that get in the way of seeing real type errors.

Removing custom errors cleans up a lot of tickets, and as a bonus, is a lot of red.

In the real world, this is a significant improvement in polysemy's code generation. When building Galley.API.Public from wireapp/wire-server#1917, this change is:

- terms: 16,086
+ terms: 16,978
- types: 7,910,353
+ types: 7,916,780
- coercions: 13,919,222
+ coercions: 5,551,380
- ppr core size: 951956810
+ ppr core size: 361974977

Notice that coercions are down by 60%, and that the core size is 1/3.

This is a breaking change and needs a major version bump.

Fixes #107
Fixes #154
Fixes #227
Fixes #363
Fixes #341
Fixes #426