Use more consistent error messages for bad flattening

serde/src/private/de.rs

@@ -2095,7 +2095,7 @@ impl<'a, 'de, E> Deserializer<'de> for FlatMapDeserializer<'a, 'de, E>
     where
         V: Visitor<'de>,
     {
-        Err(Error::custom("can only flatten structs, maps and struct enum variants"))
+        Err(Error::custom("can only flatten structs and maps"))
     }
 
     fn deserialize_enum<V>(

serde/src/private/ser.rs

@@ -1043,9 +1043,7 @@ where
     M: SerializeMap + 'a
 {
     fn bad_type(self, what: Unsupported) -> M::Error {
-        ser::Error::custom(format_args!(
-            "cannot flatten serialize {} values", what
-        ))
+        ser::Error::custom(format_args!("can only flatten structs and maps (got {})", what))
     }
 }