chore: suppress connection error when disabling an extension

[chrismwendt]

Previously, an unhandled promise rejection could occur when disabling a Sourcegraph extension. Here's a rough sequence of events that could trigger it:

• The user disables an extension in the UI
• The configuration gets updated with "foo-extension": false
• sourcegraph-extension-api sends a request over JSON-RPC (which would eventually notify foo-extension of this configuration change)
• Before the response comes back, e-c-c removes foo-extension from the list of active extensions and sourcegraph-extension-api unsubscribes the connection
• The in-flight responses are rejected
• The rejected promise turns into the console error we see

All I did was suppress the error, because this seems like a case that "shouldn't happen". It's kind of like we're sending SIGTERM to the extension but not waiting 500ms for the extension to exit.

A more involved solution could be: don't send a configuration update to an extension if the only change was the enablement state.

Another solution could be: separate the concepts of enablement state and configuration.

See https://github.com/sourcegraph/sourcegraph/issues/373

[sourcegraph-bot]

:tada: This PR is included in version 18.0.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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