Provide specific error message for common errors

[karlb]

This should make it easier to known what's going on when we get "no
route found" errors.

[codecov]

Codecov Report

Merging #674 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #674      +/-   ##
==========================================
- Coverage   91.18%   91.17%   -0.02%     
==========================================
  Files          35       35              
  Lines        2382     2401      +19     
  Branches      304      313       +9     
==========================================
+ Hits         2172     2189      +17     
- Misses        151      152       +1     
- Partials       59       60       +1

Impacted Files	Coverage Δ
src/pathfinding_service/api.py	96.7% <100%> (+0.04%)	⬆️
src/pathfinding_service/model/token_network.py	95.78% <100%> (+0.38%)	⬆️
src/raiden_libs/database.py	88.4% <0%> (-2.9%)	⬇️

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[karlb]

@hackaugusto reviewed it on my machine.