internal/impl: make errInvalidUTF8 be a proto.Error

Fixes golang/protobuf#1174.

Change-Id: If95f9f79fff76dc0c44f7ee96e484f59fa287e58
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/244718
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/impl/codec_field.go b/internal/impl/codec_field.go
index 35d4002..cb4b482 100644
--- a/internal/impl/codec_field.go
+++ b/internal/impl/codec_field.go
@@ -10,6 +10,7 @@
 	"sync"
 
 	"google.golang.org/protobuf/encoding/protowire"
+	"google.golang.org/protobuf/internal/errors"
 	"google.golang.org/protobuf/proto"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
 	preg "google.golang.org/protobuf/reflect/protoregistry"
@@ -20,6 +21,7 @@
 
 func (errInvalidUTF8) Error() string     { return "string field contains invalid UTF-8" }
 func (errInvalidUTF8) InvalidUTF8() bool { return true }
+func (errInvalidUTF8) Unwrap() error     { return errors.Error }
 
 // initOneofFieldCoders initializes the fast-path functions for the fields in a oneof.
 //