ErrorDefinition type does not have a "package" field

[nmiyake]

What happened?

The ErrorDefinition type does not have a "package" field, but error types do require a package -- currently, this means that errors always use the default-package value under the types.definitions hierarchy (and that the Conjure YAML that defines errors aren't treated as valid unless a value is specified for default-package).

What did you want to happen?

The ErrorDefinition Conjure type should have a package field that behaves analogously to all other types: if specified, its value should be used as the package for the error, and the default-package field should only be used as a fallback.