If the build() method cannot convert the validation error to the
generated builder error, this is a problem with the validation function.
The error span should point there, not at the macro call-site.

Due to some issue in `darling` or `syn`, this change is pointing at the
closing quotation instead of the whole path; that is being tracked as
TedDriggs/darling#120.

Structured errors need a safe way to act on uninitialized fields
that doesn't conflate with validation errors. This commit adds that by
providing a struct to represent an uninitialized field and generating
conversions from that into the auto-generated error type.

This test checks that we don't require unnecessary impls
for custom errors when uninitialized fields are impossible.