Answer architecture round 3: New loads only sources without an index and what they read, an entry drops reads, ErrLoad sits beside New, the manifest decoder moves to manifest.go and names each mistake once as jsonvalue.Kind words it

fejkdata.go

@@ -3,6 +3,7 @@ package fejkdata
 import (
 	crand "crypto/rand"
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"io/fs"
 	"reflect"
@@ -67,6 +68,16 @@ func WithoutShippedData() Option {
 	return func(c *config) { c.shipped = false }
 }
 
+// ErrLoad marks data that fails to load: in New, or on the first call reaching a category
+// of an indexed source.
+var ErrLoad = errors.New("data fails to load")
+
+// loadError is a load's error, unchanged in its text, matching ErrLoad.
+type loadError struct{ error }
+
+func (e loadError) Unwrap() error      { return e.error }
+func (loadError) Is(target error) bool { return target == ErrLoad }
+
 // New builds a generator from the shipped data set and the options' sources, merged
 // in order with the last winning a name clash. Each JSON file becomes a category
 // named after the file (address.json -> "address") and each subdirectory a
@@ -91,7 +102,7 @@ func New(opts ...Option) (*Generator, error) {
 	return &Generator{drawState: drawstate.New(seed), root: root}, nil
 }
 
-// load builds the tree New starts from. docs/decisions.md#a-source-whose-manifest-carries-an-index-loads-each-category-on-first-reach-any-other-loads-in-new-with-what-it-reads-and-every-indexed-category-depending-on-what-it-provides-or-a-later-source-replaced
+// load builds the tree New starts from. docs/decisions.md#a-source-whose-manifest-carries-an-index-loads-each-category-on-first-reach-any-other-loads-in-new-with-what-it-reads
 func (c config) load() (folder, error) {
 	var sources []datafiles.Source
 	if c.shipped {