[FIX] Keywords - Handle connection error

[PrimozGodec]

Issue

Fixes https://github.com/biolab/orange3-text/issues/890

Description of changes

Handle connection errors and warn users about them.

Includes

• [X] Code changes
• [X] Tests
• [ ] Documentation

[codecov-commenter]

Codecov Report

Merging #992 (9867d3c) into master (9c0faca) will increase coverage by 0.06%. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #992 +/- ## ========================================== + Coverage 79.66% 79.72% +0.06% ========================================== Files 87 87 Lines 12319 12332 +13 Branches 1617 1620 +3 ========================================== + Hits 9814 9832 +18 + Misses 2211 2209 -2 + Partials 294 291 -3 ```