Error codes could be considered a subset (i.e., an indication) of
errors, specifically in regards to HTTP errors. Errors are much more encompassing and could refer to really any sort of error however, be it an application error or otherwise...
Therefore these two tags should remain un-unmerged, even if one was used much less. However, as with many tags on this site, we really could use a wiki summary for both the error an error-code tags, for which a shiny new badge would be awarded for the effort, and you'd show up on this list :-)
More wiki summaries would really help clarify which tags to use and search on, and likely result in less confusion in cases like this where the user might just select both since they seem to be similar.