get rid of unserializable error type.

api/api_storage.go

@@ -355,12 +355,12 @@ type DealSchedule struct {
 type DagstoreShardInfo struct {
 	Key   string
 	State string
-	Error error
+	Error string
 }
 
 // DagstoreGCResult is the serialized form of dagstore.GCResult that we expose
 // through JSON-RPC to avoid clients having to depend on the dagstore lib.
 type DagstoreGCResult struct {
 	Key   string
-	Error error
+	Error string
 }

api/api_test.go

@@ -76,7 +76,7 @@ func TestReturnTypes(t *testing.T) {
 						seen[typ] = struct{}{}
 
 						if typ.Kind() == reflect.Interface && typ != bareIface && !typ.Implements(jmarsh) {
-							t.Error("methods can't return interfaces", m.Name)
+							t.Error("methods can't return interfaces or struct types not implementing json.Marshaller", m.Name)
 						}
 
 						switch typ.Kind() {

api/docgen/docgen.go

@@ -278,12 +278,12 @@ func init() {
 	})
 	addExample(api.DagstoreGCResult{
 		Key:   "baga6ea4seaqecmtz7iak33dsfshi627abz4i4665dfuzr3qfs4bmad6dx3iigdq",
-		Error: nil,
+		Error: "<error>",
 	})
 	addExample(api.DagstoreShardInfo{
 		Key:   "baga6ea4seaqecmtz7iak33dsfshi627abz4i4665dfuzr3qfs4bmad6dx3iigdq",
 		State: "ShardStateAvailable",
-		Error: nil,
+		Error: "<error>",
 	})
 }