Fix a number of failures that meant the excellent error handling
we had for failing to restore a session didn't work.
1. .catch on the promise rather than try/catch: it's async
2. Explicit cancel method in SessionRestoreErrorDialog that invokes
onFinished with `false` because even with the catch fixed, this
was getting the event as its first arg which is truthy, so
clicking cancel still deleted your data.
3. DialogButtons: Don't pass onCancel straight into the button event
handler as this leaks the MouseEvent through as an argument.
Nothing is using it and it exacerbates failures like this
because there are surprise arguments.
Fixes https://github.com/vector-im/riot-web/issues/6616
