fix: change exists to return false on error code 5

[callmehiphop]

Fixes #326

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3d47e54). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #353   +/-   ##
=========================================
  Coverage          ?   99.53%           
=========================================
  Files             ?       12           
  Lines             ?     1300           
  Branches          ?        0           
=========================================
  Hits              ?     1294           
  Misses            ?        6           
  Partials          ?        0

Impacted Files	Coverage Δ
src/database.js	97.45% <100%> (ø)
src/instance.js	100% <100%> (ø)

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