[Fixed] Better error handling on billing and publishing pages

Fixes

• Failed credit top-ups or job boost requests no longer redirect you to an "undefined" page.
• Boost checkout now shows a clear error message when a payment cannot be started instead of failing silently.
• Disqualifying a candidate no longer crashes the page when stages are still loading.
• Publishing a sponsored campaign no longer displays phantom campaign details after a server error.
• Loading spinners now clear correctly after any server error on the affected pages, preventing the interface from getting stuck.