Inform user of failed sidebar injection

[fregante]

We used to display an alert when the sidebar would not open, but due to long delays, it caused confusion.

Now if the sidebar fails to load, absolutely nothing happens.

Proposed solutions

• [ ] in background: show alert when content scripts doesn't respond, probably in ensureContentScript
• [ ] in content script: show notification error when sidebar doesn't respond

Related

• https://github.com/pixiebrix/pixiebrix-extension/pull/517
• https://github.com/pixiebrix/pixiebrix-extension/pull/4079#event-7236762344
• https://github.com/pixiebrix/pixiebrix-extension/issues/4021
• https://github.com/fregante/webext-dynamic-content-scripts/issues/41
• #4125

[fregante]

While it doesn't change the outcome of a failure, a recent PR should at least give immediate feedback when the browserAction is clicked, even if it eventually fails:

• https://github.com/pixiebrix/pixiebrix-extension/pull/4099

For this reason, in similar tone to #4117, I think the sidebar should be in a "Connecting…" state until it receives a message.