Update Module generators: Improve error when mender-artifact not present

Most Update Modules come with a Mender Artifact generator that uses the `mender-artifact` utility under the hood. If `mender-artifact` is not installed an error is generated, however this can be confusing for the user (why is it complaining about mender-artifact, and how do I fix it?).

Here is a sample question:

• Q: Running mender-binary-delta-generator it complains it does not find mender-artifact. Any ideas?
• A: Do you have the mender-artifact binary in your PATH? https://docs.mender.io/downloads#mender-artifact

We should improve the error message so that such questions don't need to be asked, but it is instead clear why it happens and how to fix it.

This applies to mender-binary-delta-generator but also to most Update Modules that we have created and we should cover it in all of them.

Acceptance criteria

• If mender-artifact is not installed, the Artifact generator exits with an error message explaining how to install it
• All Update Module generators in Mender Hub are adjusted to this (see below)

The following are Update Modules that have a generator:

• https://hub.mender.io/t/directory-overlay
• https://hub.mender.io/t/reboot-remotely
• https://hub.mender.io/t/single-file
• https://hub.mender.io/t/docker
• https://hub.mender.io/t/directory
• https://hub.mender.io/t/robust-delta-update-rootfs
• https://hub.mender.io/t/device-firmware-update-dfu

Suggested error message:

The mender-artifact utility is not found but required to generate Artifacts.
Please follow the instructions here to install it and then try again: https://docs.mender.io/downloads#mender-artifact