Most places used to call `context.error()` to report an error,
which would set the context's state to `Error` and then return an
`Error::from_string_literal()`.
This is somewhat elegant, but it doesn't work: Some functions this
code calls returns ErrorOr<>s that aren't created by `context.error()`,
and for these we wouldn't enter the error state.
Instead, manually check error-ness at the leaf entry functions of the
class:
1. Add a set_error() helper for functions returning bool
2. In the two functions returning ErrorOr<>, awkwardly check the error
manually. If this becomes a very common pattern, maybe we can add
a `TRY_WITH_HANDLER(expr, error_lambda)` which would invoke a
lambda on error. We could use that here to set the error code.
No real behavior change (except we enter the error state more often
when something goes wrong).
