Better handling if Server API has problems

[MTRNord]

https://sentry.io/nordgedanken/socialnetworksnews/issues/473760307/
Public: https://sentry.io/share/issue/c3265796685f43a993c32320c56525af/

Error: {"headers":{"normalizedNames":{},"lazyUpdate":null},"status":500,"statusText":"Internal Server Error","url":"https://freifunknews.nordgedanken.blog/api/paper/5f0487de-87e8-4b7e-ba70-9d77248d2c8e/yesterday","ok":false,"name":"HttpErrorResponse","message":"Http failure response for https://freifunknews.nordgedanken.blog/api/paper/5f0487de-87e8-4b7e-ba70-9d77248d2c8e/yesterday: 500 Internal Server Error","error":"open data/tweets_2018_02_25.csv: no such file or directory\n"}
  at fn (webpack:/C:/Users/marce/WebstormProjects/SocialNetworksNews-Web2/src/app/tweet-list/tweet-list.component.ts:33:39)
  at this (webpack:/C:/Users/marce/WebstormProjects/SocialNetworksNews-Web2/node_modules/rxjs/_esm5/Subscriber.js.pre-build-optimizer.js:240:13)
  at this (webpack:/C:/Users/marce/WebstormProjects/SocialNetworksNews-Web2/node_modules/rxjs/_esm5/Subscriber.js.pre-build-optimizer.js:199:21)
  at this (webpack:/C:/Users/marce/WebstormProjects/SocialNetworksNews-Web2/node_modules/rxjs/_esm5/Subscriber.js.pre-build-optimizer.js:131:9)
  at this (webpack:/C:/Users/marce/WebstormProjects/SocialNetworksNews-Web2/node_modules/rxjs/_esm5/Subscriber.js.pre-build-optimizer.js:105:13)
...
(16 additional frame(s) were not displayed)

[MTRNord]

18ffb5b <-- this propably fixes it partialy