Adds showing the error screen when API is down

[manuelwedler]

Closes #18
Fixes #190

[codecov]

Codecov Report

Merging #210 into master will increase coverage by 0.2%.
The diff coverage is 96.29%.

@@            Coverage Diff            @@
##           master     #210     +/-   ##
=========================================
+ Coverage   86.47%   86.67%   +0.2%     
=========================================
  Files          82       82             
  Lines        1811     1839     +28     
  Branches      242      247      +5     
=========================================
+ Hits         1566     1594     +28     
  Misses        199      199             
  Partials       46       46

Impacted Files	Coverage Δ
...c/app/services/pending-transfer-polling.service.ts	97.22% <ø> (ø)	⬆️
...nents/payment-history/payment-history.component.ts	93.47% <ø> (ø)	⬆️
src/app/services/channel-polling.service.ts	100% <ø> (ø)	⬆️
...omponents/token-network/token-network.component.ts	48.62% <ø> (ø)	⬆️
src/app/components/error/error.component.ts	100% <100%> (ø)	⬆️
src/app/interceptors/error-handling.interceptor.ts	89.18% <100%> (+1.68%)	⬆️
src/app/services/raiden.service.ts	90.52% <100%> (+0.47%)	⬆️
src/app/services/notification.service.ts	100% <100%> (ø)	⬆️
src/app/services/raiden.config.ts	96% <100%> (+0.16%)	⬆️
src/testing/mock-config.ts	81.81% <100%> (ø)	⬆️
... and 2 more

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[andrevmatos]

lgtm, thank you!