Fix "can not convert object to primitive" nonsence error message

[secam]

MikroOrm that uses umzug under the hood, throws custom errors in the form of objects derived from null (Null prototype objects) that are not handled correctly by errorString method, this result in confusing error message masking original error completely (nothing related to original error in stack trace)
This micro fix solves this and makes errorString more universal in handling such tricky objects.

[pkg-pr-new]

Open in Stackblitz

pnpm add https://pkg.pr.new/sequelize/umzug@697

commit: 0715f1f

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (7dc3877) to head (0715f1f).
Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #697   +/-   ##
=======================================
  Coverage   99.45%   99.45%           
=======================================
  Files          12       12           
  Lines        1466     1473    +7     
  Branches      269      269           
=======================================
+ Hits         1458     1465    +7     
  Misses          8        8           

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