Await key cache check to avoid prompts
Await key cache check to avoid prompts

`isStoredInKeyCache` is async, so we need to await the result to know whether to
proceed. In addition, this wraps the block in a try / catch, since signing the
key backup is an optional step.

Fixes https://github.com/vector-im/element-web/issues/15530

matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk