Sprint goals are the result of a negotiation between the product owner and the development team. Meaningful goals are specific and measurable. Instead of "Improve scalability" try "Handle five times as many users as version 0.8."
Scrum focuses on goals that result in demonstrable product. The product owner is entitled to expect demonstrable product (however small or flimsy) starting with the very first Sprint. In iterative development, subsequent Sprints can increase the robustness or size of the feature set.
Have your team commit to goals that anyone will be able to see are met (or not met) at the end of the sprint. At sprint review meetings, the sprint demonstration is conducted after which the team asks the product owner whether (s)he feels the goals were met.
While some specific product backlog items may not be done at the end of a sprint, it should be very unusual for a team not to meet its sprint goals. Scrum requires the team to notify the product owner as soon as it becomes aware it will not meet its goals.