Align ErrorState usage by removing redundant div wrappers

What does this MR do and why?

Several pages wrapped <ErrorState /> in a redundant element: DashboardPage, ActivityDashboardPage, and ManageMergeRequestPage used <div v-if="error">, and ActivitiesPage used <template v-if="error">. The wrapper added nothing, and it was inconsistent with how other pages render the component.

This moves the v-if="error" condition directly onto <ErrorState> in all four pages, so the error state is rendered the same way everywhere. It is a structural cleanup only. The rendered output is unchanged, and there is no behavioural or user-facing change.

Related Issues

Closes #661 (closed)

Validation steps

Open each affected page (Dashboard, Activities, Activity Dashboard, Manage Merge Request) and force its data fetch to fail so the error branch renders. Confirm the ErrorState component still displays as before, with no extra wrapper element around it. The non-error branch is unchanged.