In the InDesign SDK, you will find errorcodes.html that would list all error codes with string equivalents.
You can perhaps download the InDesign server SDK and find inside docs folder.
I did download the InDesgin CS6 Server SDK from here | Adobe Developer Connection . As you said, it does have an errorcodes.htm under /docs/references/ directory. But the the file is empty (no kidding). Adobe needs to update the documentation. I thought the Windows SDK might have the codes, still the same just empty html file. If you have that file with error codes listed, would you mind posting it.
Thanks a lot! Much appreciated.