Display error message when dashboard activity fetch fails

What does this MR do?

Refactors Pager.init to include error handling capabilities. We then use this to createFlash when activity fetching fails.

This MR will also resolve #262588 (closed)

Screenshots (strongly suggested)

Current behavior: when request fails, loading spinner disappears with no error message.

This MRs behaviour: when request fails, loading spinner disappears and generic error message is displayed.

Before (dashboard activity)	After (dashboard activity)
before-error-handler	after-error-handler

User activity	Project activity	Group activity

Does this MR meet the acceptance criteria?

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