Right now the Twitter API simply returns user-readable textual descriptions of error conditions, and 
there is no list that I know of that enumerates the possible errors a client might encounter.  This 
makes things difficult for client developers who want to display friendlier error messages or 
introduce special error handling logic in their applications.  Basically, if you want to do anything 
except display the raw API error message to the user, you have to key your error handling logic to 
strings that seem like they could change and aren't necessarily consistent across request types (for 
example "Not authorized" in some places "Sorry, you are not authorized to see this status." in 
others).  I think this could all be solved by the use of error codes, and the enumeration of the 
possible error codes an API method can return in the documentation.