Explicitly guard store usage during sync startup
This adds explicit `try` blocks in the spots where we interact with the store
during sync startup. This shouldn't be necessary as the store should already be
catching this and degrading as of
https://github.com/matrix-org/matrix-js-sdk/pull/884, but that doesn't seem to
have been enough for the affected user in
https://github.com/vector-im/riot-web/issues/7769, as they are seeing sync just
stop when storing without any further detail.
