[Developers] Freebase API Error response format

[Brian Karlak]

> 3) Messages.  Each returned message has a human-readable string (in
> English) and a detailed code value.  Our intention is for the detailed
> code values to be useful in looking up translated error messages.

Is there a standard list for the expected error code values?  There  
are definitely cases in which we need to choose different execution  
paths depending on the returned MQL error code (eg, "value exists, try  
update" errors).

A list of the standard error codes would help in implementing and  
testing these cases.

Brian