Chatserver should always respond to requests

Summary

Apparently there is bug in tedis: https://github.com/silkjs/tedis/issues/49

What is the current bug behavior?

Currently this leads only to an error in the console, but requests stay open forever which led to the foodsharing outage 2021-03-30

What is the expected correct behavior?

On an error (not just ones caused by tedis!), the chatserver should always respond with an 5xx error.

Relevant error messages

 error: Error: This socket has been ended by the other party                                                                                                            
     at Socket.writeAfterFIN [as write] (net.js:455:14)                                                                                                                 
     at /var/www/production/releases/31/chat/node_modules/tedis/build/core/base.js:83:26                                                                                
     at new Promise (<anonymous>)
     at Tedis.Base.command (/var/www/production/releases/31/chat/node_modules/tedis/build/core/base.js:78:16)                                                           
     at SessionIdProvider.fetchSessionIdsForUser (/var/www/production/releases/31/chat/src/SessionIdProvider.ts:24:43)                                                  
     at runMicrotasks (<anonymous>)
     at processTicksAndRejections (internal/process/task_queues.js:97:5)                                                                                                
     at RestController.userIsConnected (/var/www/production/releases/31/chat/src/RestController.ts:26:28) {                                                             
   code: 'EPIPE'
 }