[ui] Single file artifact generation allows proceeding without a device type and with wrong path

The UI dialog to upload a file and generate a single-file artifact doesn't require the device type. This field is mandatory, though, and the absence of a value makes the server-side workflow to fail. Similarly, the dialog warns the user about the need of an absolute path, but allows him/her to proceed with a relative path. This, again, makes the server-side workflow to fail.

The failure server-side is handled, but it would be better to guide the user to avoid these mistakes given the fact the single-file artifact generation tool is targeted to point-and-click users.