Before this change, if RetryAfterError was called with a nil err, then
it's Error method would return this when wrapped in a fmt.Errorf
statement
error %!v(PANIC=Error method: runtime error: invalid memory address or nil pointer dereference))
Looking at the code, it looks like RetryAfterError will usually be
called with a nil pointer, so this patch makes sure it has a sensible
error.
79bd9e7913