Consider printing a command line warning when exporting SVG's if dependencies are not present

[bparks13]

When automatically generating an SVG of a workflow, using --export-image from the command line, there is no warning currently if a dependency for the given workflow is not loaded. Instead, the resulting SVG includes a node with cross-hatches.

It would be useful to know that an image would be missing node information, but specifically this would be useful for automating the export of workflows to create documentation. Having some way to check that the command worked instead of silently generating the image with X-ed out nodes would be beneficial in GitHub Actions workflows to point out when manual intervention might be necessary.

This seems tangentially related to #1725 as well.