opt:Error message for optimizing mapping data method

[R1aEnKK]

Writing the wrong mapping field clearly helps troubleshoot the problem

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.56%. Comparing base (8690859) to head (36004d6).
Report is 213 commits behind head on master.

Additional details and impacted files

Files with missing lines	Coverage Δ
core/conf/config.go	98.66% <100.00%> (+0.01%)	⬆️

... and 8 files with indirect coverage changes