chore: improve error logs during processor stash test

[achettyiitr]

Description

• Currently, we are logging as a byte sequence. GA link.

    stash_test.go:159: file list:  [0xc0007a10b0 0xc0007a10e0]  err:  <nil> len:  2
    stash_test.go:159: file list:  [0xc000bb83f0 0xc000bb8420]  err:  <nil> len:  2
    stash_test.go:153: 
        	Error Trace:	/home/runner/work/rudder-server/rudder-server/processor/stash/stash_test.go:153
        	Error:      	Condition never satisfied
        	Test:       	TestStoreErrorsToObjectStorage
        	Messages:   	no backup files found in backup store: %!(EXTRA <nil>)
--- FAIL: TestStoreErrorsToObjectStorage (26.17s)

• Log as string instead of byte sequence for better debuggability.

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2b82920) 74.06% compared to head (ded16f7) 74.08%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4317      +/-   ##
==========================================
+ Coverage   74.06%   74.08%   +0.01%     
==========================================
  Files         390      390              
  Lines       55274    55274              
==========================================
+ Hits        40940    40947       +7     
+ Misses      12006    12000       -6     
+ Partials     2328     2327       -1     

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