chore: correct cast from int to string in error message formatting

[acastle]

Small behavioral bug that has been around for a while. In the error message we are casting the index to a rune resulting in text characters rather than the numeric string.

[codecov]

Codecov Report

Merging #1327 (367f9f0) into master (06991d7) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1327   +/-   ##
=======================================
  Coverage   70.05%   70.05%           
=======================================
  Files          74       74           
  Lines        5540     5540           
=======================================
  Hits         3881     3881           
  Misses       1303     1303           
  Partials      356      356           

Impacted Files	Coverage Δ
server/events/db/boltdb.go	70.65% <0.00%> (ø)

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