Block updates in web UI when the read-only filesystem is enabled

[mtlynch]

If the user enabled the read-only filesystem and then performs an update, the system will reboot and go back to the pre-update state, which is not what the user wants or expects.

The update dialog should show an error message if the user attempts to update while the read-only filesystem is enabled.

[db39]

A user via Helpscout attempted to update their device and ran into an update error. Their device had the read-only filesystem enabled. This feature would have helped prevent the update from starting and let the user know what they needed to do to perform the update.