Keep partially downloaded index with resume option if synchronization stops or fails

[gtandersen]

When synchronizing the message index the download may stop or fail for various reasons such as network issues.

In this case any fully downloaded index partitions should be retained in the browser so that the user may resume the rest of the download (instead of starting from scratch) and use the partial index in the meantime.

[gtandersen]

This implies that there could be a "Resume index synchronization" button somewhere in the UI.