Fallback to document.getSelection() on read permission prompt as intended instead of consuming NotAllowedError

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Description

When options.read === true and the 'clipboard-read' permission value is 'prompt', thus isAllowed() returns true and an attempt to use the Clipboard API is made, before the user can press either Allow or Deny, the Promise returned by navigator.clipboard.readText() can be rejected with a NotAllowedError. This was covered in #4512, but a wrong syntax combination was used, consequently, the fallback to legacyRead() didn't happen, and an unhandled rejection with a NotAllowedError was generated in the console. This PR handles the rejection by falling back as intended.

Additional context

See reply to where this was introduced. Async/await is supported by addEventHandler() and was preferred to Promise.catch() for its advantages and by analogy to copy() from the same PR.