Show different message for Preview

[nataliemt]

Every time it is not possible to load a preview, the following message is displayed:

The Preview is loaded when all required fields are filled. Although the message shows that it was not possible to load the preview section, it is not clear that the reason is because of one of the mandatory fields was not entered.
The above message should be displayed as default when the page is loading but once user start filling the fields, the message should be:
"Please, select all required fields to generate a preview"

[pestevez]

I believe the text "connect the required fields" is the one that we are using (based on the tool's jargon) to reflect that there is at least one mandatory field not entered.