Users are missing messages when their Slack token expires

[tadzik]

If, for any reason, user's Slack token expires (giving us a this error:

WARN SlackClientFactory Failed to auth puppeted client for user: Error: An API error occurred: token_revoked
    at Object.platformErrorFromResult (/usr/src/app/node_modules/@slack/web-api/dist/errors.js:51:33)
    at WebClient.apiCall (/usr/src/app/node_modules/@slack/web-api/dist/WebClient.js:156:28)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at async SlackClientFactory.getClientForUserWithId (/usr/src/app/lib/SlackClientFactory.js:182:26)
    at async SlackClientFactory.getClientForUser (/usr/src/app/lib/SlackClientFactory.js:200:21)
    at async Main.startupLoadRoomEntry (/usr/src/app/lib/Main.js:1041:31)
    at async /usr/src/app/lib/Main.js:986:17
    at async Promise.all (index 659)
    at async Main.run (/usr/src/app/lib/Main.js:983:9) {
  code: 'slack_webapi_platform_error',
  data: { ok: false, error: 'token_revoked', response_metadata: {} }
}

), we're not notifying users in any way – they will still see messages on public channels, but not in DMs – making them feel like the bridge is broken rather than assuming that they lost access. We should be handling these errors properly, notifying the affected users that they need to re-login to Slack to be able to use the bridge.

matrix-org/matrix-appservice-bridge#400 may be a useful prerequisite for this.