Fix stuck drag on off-frame mouse-up, add error logging and slash commands

- Remove dead VARIABLES_LOADED registration (no handler existed for it)
- Extract OctoLatency_StopMoving and call it from OnHide too, so a drag
  can't get stuck if OnMouseUp never fires on the frame
- Wrap tooltip/update/login handlers in pcall, logging failures to
  OctoLatencyDB.errorLog instead of raising a Lua error popup
- Add /octolat errors and /octolat clearerrors slash commands
- Update README with a short explanation of the addon and its options

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

OctoLatency.xml

@@ -47,12 +47,13 @@
 				end
 			</OnMouseDown>
 			<OnMouseUp>
-				if (arg1 == "LeftButton" and this.isMoving) then
-					this:StopMovingOrSizing();
-					this.isMoving = false;
-					OctoLatency_SavePosition();
+				if (arg1 == "LeftButton") then
+					OctoLatency_StopMoving(this);
 				end
 			</OnMouseUp>
+			<OnHide>
+				OctoLatency_StopMoving(this);
+			</OnHide>
 		</Scripts>
 	</Frame>
 </Ui>