fix(slack): dont panic on send http errors

[piksel]

ref: containrrr/watchtower#758

[codecov]

Codecov Report

Merging #114 (483ada4) into main (1290cec) will increase coverage by 1.10%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
+ Coverage   52.79%   53.90%   +1.10%     
==========================================
  Files          55       55              
  Lines        1161     1165       +4     
==========================================
+ Hits          613      628      +15     
+ Misses        512      497      -15     
- Partials       36       40       +4     

Impacted Files	Coverage Δ
pkg/services/slack/slack.go	63.63% <60.00%> (+41.41%)	⬆️
pkg/services/slack/slack_json.go	100.00% <0.00%> (+100.00%)	⬆️

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