Enhance load-error event handling for ZuoraSimple Component

Problem: As per this MR review comment, we should inform the user with a proper message in case the iframe in ZuoraSimple component fails to load as this is a terminal state for the user and the only way they can try to proceed is by refreshing the page.

Also right now the load-error is not captured when the zuora script fails to load. We should emit load-error event when the zuora script fails to load.

Proposal:

• Update the error message in case of Iframe load error to inform user to refresh the page. We can get the exact wordings of the message from UX.
• Add a load-error event in case the zuora script fails to load. (similar to the onload handling we should add onerror handling.)

Note: This a follow up to Zuora Simple Component: implement events API (#358782 - closed) as per this MR review comment.