fix: properly handle unreachable network

[JKRhb]

Hopefully resolves #74.

[codecov-commenter]

Codecov Report

Patch coverage: 54.28% and project coverage change: -0.68 :warning:

Comparison is base (fe5eecf) 84.10% compared to head (233981f) 83.42%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #75 +/- ## ========================================== - Coverage 84.10% 83.42% -0.68% ========================================== Files 10 11 +1 Lines 717 724 +7 ========================================== + Hits 603 604 +1 - Misses 114 120 +6 ``` | [Impacted Files](https://codecov.io/gh/JKRhb/dtls2/pull/75?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann) | Coverage Δ | | |---|---|---| | [lib/src/dtls\_connection.dart](https://codecov.io/gh/JKRhb/dtls2/pull/75?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann#diff-bGliL3NyYy9kdGxzX2Nvbm5lY3Rpb24uZGFydA==) | `0.00% <0.00%> (ø)` | | | [lib/src/dtls\_server.dart](https://codecov.io/gh/JKRhb/dtls2/pull/75?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann#diff-bGliL3NyYy9kdGxzX3NlcnZlci5kYXJ0) | `87.22% <54.54%> (-0.28%)` | :arrow_down: | | [lib/src/dtls\_client.dart](https://codecov.io/gh/JKRhb/dtls2/pull/75?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann#diff-bGliL3NyYy9kdGxzX2NsaWVudC5kYXJ0) | `78.12% <61.90%> (-0.80%)` | :arrow_down: |

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