refactor: Separate use cases of URLNotFound and ObjectNotFound

[achimnol]

refs lablup/backend.ai#378

[codecov]

Codecov Report

Merging #581 (3950393) into main (74ec17c) will increase coverage by 0.01%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##             main     #581      +/-   ##
==========================================
+ Coverage   48.57%   48.58%   +0.01%     
==========================================
  Files          53       53              
  Lines        9127     9133       +6     
==========================================
+ Hits         4433     4437       +4     
- Misses       4694     4696       +2     

Impacted Files	Coverage Δ
src/ai/backend/manager/api/auth.py	48.44% <0.00%> (ø)
src/ai/backend/manager/server.py	57.53% <0.00%> (ø)
src/ai/backend/manager/api/events.py	38.24% <50.00%> (ø)
src/ai/backend/manager/api/exceptions.py	87.83% <75.00%> (-0.59%)	⬇️

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