Throw error when API_URI is not set

[dominik-zeglen]

I want to merge this change because it resolves #102

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. Translated strings are extracted to .pot file.
4. Number of API calls is optimized.
5. The changes are tested.
6. Type definitions are up to date.
7. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #375 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #375   +/-   ##
=======================================
  Coverage   72.88%   72.88%           
=======================================
  Files         612      612           
  Lines        6834     6834           
  Branches     1139     1138    -1     
=======================================
  Hits         4981     4981           
  Misses       1620     1620           
  Partials      233      233

Impacted Files	Coverage Δ
src/config.ts	100% <100%> (ø)	⬆️

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