Improve error message when no rootfs-image support

This has come up a few times, especially in cases where only the Mender debian package is installed (not full integration) or some configuration is wrong.

When trying to deploy an update of the "rootfs-image" type, the following error is shown:

“Fetching Artifact headers failed: installer: failed to read Artifact: readHeaderV3: handleHeaderReads: Cannot load handler for unknown Payload type 'rootfs-image'"

If you know the details this makes sense (no Update Module or internal logic to understand the 'rootfs-image' type), but for a user this can be very confusing. Is there something wrong with the Artifact Payload or the Device? If Device, what is wrong and what should be done to fix it?

I would recommend two changes:

• In general, instead of "..Cannot load handler for unknown Payload type 'rootfs-image'" -> "Artifact Payload type 'rootfs-image' is not supported by this Mender client."
• Specifically for "rootfs-image" type, we can add: "Ensure Mender client is fully integrated, including RootfsPartA/B is set in mender.conf."