fix: Minor improvement in error handling

[jan-auer]

Two unrelated improvements that were found while working on other features:

1. Improve the expect when creating the backend HTTP client. This will panic
   regardless, but improves the error message.
2. Log a warning when parsing the Sentry DSN fails. This makes it easier to
   investigate when no data turns up in Sentry.

[codecov]

Codecov Report

❌ Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.62%. Comparing base (181df0a) to head (ae9b45d).

Files with missing lines	Patch %	Lines
objectstore-server/src/observability.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #576      +/-   ##
==========================================
- Coverage   87.64%   87.62%   -0.03%     
==========================================
  Files          94       94              
  Lines       15425    15430       +5     
==========================================
  Hits        13520    13520              
- Misses       1905     1910       +5     

Components	Coverage Δ
Rust Backend	92.31% <14.28%> (-0.04%)	⬇️
Rust Client	79.89% <ø> (ø)
Python Client	90.90% <ø> (ø)

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