Give the help examples that read no data and the exit codes, say a data path is no directory in one shape, suggest a record of one for a choice, name a JSON syntax error's line and column, and say no data is loaded from FakeStruct

internal/datafiles/datafiles.go

@@ -2,6 +2,7 @@
 package datafiles
 
 import (
+	"errors"
 	"fmt"
 	"io/fs"
 	"os"
@@ -91,15 +92,25 @@ func (s Source) check() error {
 		return fmt.Errorf("a data path is empty")
 	}
 	info, err := os.Stat(s.label)
+	if errors.Is(err, fs.ErrNotExist) {
+		return noDirectory(s.label)
+	}
 	if err != nil {
 		return fmt.Errorf("data path %s: %w", s.label, err)
 	}
 	if !info.IsDir() {
-		return fmt.Errorf("%s is not a directory", s.label)
+		return fmt.Errorf("data path %s: not a directory", s.label)
 	}
 	return nil
 }
 
+// noDirectory is a data path naming nothing, which errors.Is still matches as fs.ErrNotExist.
+type noDirectory string
+
+func (d noDirectory) Error() string { return "data path " + string(d) + ": no such directory" }
+
+func (noDirectory) Unwrap() error { return fs.ErrNotExist }
+
 // walkDir walks the folder that dir names, and reports whether it handed over any category.
 func (s Source) walkDir(dir []string, compile func(Category) error) (bool, error) {
 	full := s.dirPath(dir)

internal/jsonvalue/jsonvalue.go

@@ -4,7 +4,9 @@ package jsonvalue
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"unicode/utf8"
 )
 
 // Value is a JSON value that only Decode, String and Object make, so a template never compiles
@@ -15,7 +17,7 @@ type Value struct{ v any }
 func Decode(b []byte) (Value, error) {
 	var v any
 	if !json.Valid(b) {
-		return Value{}, json.Unmarshal(b, &v)
+		return Value{}, located(b, json.Unmarshal(b, &v))
 	}
 	dec := json.NewDecoder(bytes.NewReader(b))
 	dec.UseNumber()
@@ -23,6 +25,18 @@ func Decode(b []byte) (Value, error) {
 	return Value{v}, err
 }
 
+// located prefixes a syntax error with the line and column of the byte it stopped at.
+func located(b []byte, err error) error {
+	var syntax *json.SyntaxError
+	if !errors.As(err, &syntax) {
+		return err
+	}
+	before := b[:max(syntax.Offset-1, 0)]
+	line := bytes.Count(before, []byte("\n")) + 1
+	column := utf8.RuneCount(before[bytes.LastIndexByte(before, '\n')+1:]) + 1
+	return fmt.Errorf("line %d, column %d: %w", line, column, err)
+}
+
 // String is the JSON string s.
 func String(s string) Value { return Value{s} }