message: Return FailedPrecondition if ReadVariables is given unintialized vars
  This makes it consistent with ReadVariable.
diff: @@ -188,9 +188,9 @@ void ReadVariablesOp::Compute(OpKernelContext* ctx) {
  }
  }
  - OP_REQUIRES(
  - ctx, uninitialized_vars.empty(),
  - errors::InvalidArgument("In ReadVariableOp the following variables were "
  + OP_REQUIRES(ctx, uninitialized_vars.empty(),
  + errors::FailedPrecondition(
  + "In ReadVariablesOp the following variables were "
  "found uninitialized: ",
  absl::StrJoin(uninitialized_vars, ", ")));
  