#4124: Inform user of failed sidebar injection + handle repeated browser-action clicks

[fregante]

What does this PR do?

• Avoids attempting multiple simultaneous injections in the same tab
• Shows an error when the sidebar was supposed to open, but failed
  • Note: this PR covers an actual failure to open the sidebar rather than "unable to edit special browser pages" like https://github.com/pixiebrix/pixiebrix-extension/pull/4188

Specifically:

• Closes #4124
• https://github.com/pixiebrix/pixiebrix-extension/issues/4125

Discussion

• The "blank slate" design of the sidebar is being handled in other tickets. I think we can use the sidebar to display any connection issues, rather than injection (which at this point is unlikely)

Future Work

• [ ] #4193

Checklist

• [x] Designate a primary reviewer @twschiller