add M_UNKNOWN_POS to error constants

[mdnight]

This pull request includes a small change to the spec/matrixerror.go file. The change adds a new error code ErrorUnknownPos to the MatrixErrorCode type. This error is used by the simplified sliding sync

• spec/matrixerror.go: Added ErrorUnknownPos to the MatrixErrorCode type.

https://github.com/matrix-org/matrix-spec-proposals/blob/erikj/sss/proposals/4186-simplified-sliding-sync.md#connections

Pull Request Checklist

• Pull request includes a sign off

Signed-off-by: Roman Isaev <[email protected]>

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.43%. Comparing base (51ff16e) to head (0911849).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #448   +/-   ##
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  Coverage   59.43%   59.43%           
=======================================
  Files          51       51           
  Lines        7257     7257           
=======================================
  Hits         4313     4313           
  Misses       2544     2544           
  Partials      400      400           

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