Change log level of overflow rescanner when directory doesn't exist

[sungshik]

Before this PR, when the overflow rescanner tried to scan a directory/file that doesn't exist, a message would be logged at ERROR level. This is a bit confusing to users, as it isn't really an error but a normal situation that can happen (and is harmless; just skipping the directory/file in that case, as is done, is fine). After this PR, these messages are logged at DEBUG level.

Fixes #79

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.5%. Comparing base (ce46e33) to head (e0f21ac).
⚠️ Report is 59 commits behind head on main.

Files with missing lines	Patch %	Lines
...ing/swat/watch/impl/overflows/BaseFileVisitor.java	0.0%	3 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.0%) is below the target coverage (50.0%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #80     +/-   ##
=========================================
- Coverage     81.9%   79.5%   -2.4%     
- Complexity     166     176     +10     
=========================================
  Files           23      23             
  Lines          827     774     -53     
  Branches        97      89      -8     
=========================================
- Hits           678     616     -62     
- Misses          90      96      +6     
- Partials        59      62      +3     

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