Make caller nillable

[Bibob7]

Make caller nil-able in stack trace, since unknown error types have no caller

[codecov-commenter]

Codecov Report

Merging #3 (8c74e83) into main (b4dd3ba) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main        #3   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines           93        93           
=========================================
  Hits            93        93           

Impacted Files	Coverage Δ
error.go	100.00% <100.00%> (ø)
logging.go	100.00% <100.00%> (ø)
stack.go	100.00% <100.00%> (ø)

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