Clearer final objects error; fixes #13256 (#13257)

Author: metagn

compiler/semtypes.nim

@@ -851,7 +851,8 @@ proc semObjectNode(c: PContext, n: PNode, prev: PType; isInheritable: bool): PTy
  else:
  if concreteBase.kind != tyError:
  localError(c.config, n[1].info, "inheritance only works with non-final objects; " &
- "to enable inheritance write '" & typeToString(realBase) & " of RootObj'")
+ "for " & typeToString(realBase) & " to be inheritable it must be " &
+ "'object of RootObj' instead of 'object'")
  base = nil
  realBase = nil
  if n.kind != nkObjectTy: internalError(c.config, n.info, "semObjectNode")