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 79.4%. Comparing base (c482cb0) to head (50e52a7).

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

Additional details and impacted files

@@           Coverage Diff           @@
##              main     #80   +/-   ##
=======================================
  Coverage     79.4%   79.4%           
+ Complexity     176     175    -1     
=======================================
  Files           23      23           
  Lines          774     774           
  Branches        89      89           
=======================================
  Hits           615     615           
  Misses          96      96           
  Partials        63      63           

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