chore: log cluster sync error

[crenshaw-dev]

Right now we only pass this up to the caller to log. That has two downsides:

1. The caller may not log it (there's at least one such case in Argo CD)
2. The caller may log it too often, producing duplicate log lines and making it difficult to determine exactly how many clusters are "sick" - for example, Argo CD logs this error for every single Application deployed to the sick cluster

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.3% Duplication

[leoluz]

LGTM

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b0fffe4) 54.61% compared to head (4d728d6) 54.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #541      +/-   ##
==========================================
- Coverage   54.61%   54.60%   -0.02%     
==========================================
  Files          41       41              
  Lines        4640     4641       +1     
==========================================
  Hits         2534     2534              
- Misses       1910     1911       +1     
  Partials      196      196              

Files	Coverage Δ
pkg/cache/cluster.go	49.28% <0.00%> (-0.07%)	⬇️

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