Add specific "paid plan required" error message

[ChrisLoer]

Since Felt changed to requiring a paid plan for all data uploads, QGIS users on free accounts have been receiving a very generic "an error occurred" message when they try to upload and get a 403.

This PR watches for 403's and reports a specific error message.

Test plan:

• Install the new plugin from the zip
• Upload layers from a map using a paid workspace, make sure it works as before
• Upload layers from using a free-plan workspace, verify that you get the new more-informative error message.

[github-actions]

Plugin ready!

A test version of this PR is available for testing here.

(Built from commit c673606)

[drewhgood]

I was able to upload a layer from QGIS with a paid plan. When I tried with a free one I saw the new error message. The link to pricing and mailto links work as expected.

Code changes look correct.