Design a way to expose internal error states for debugging

As is appropriate for libudpard, the error system exposed to applications is very simple. There are times, however, when deeper introspection would be helpful. For example, when first integrating with a system the integrator is likely to make a few mistakes that are atypical for a healthy system but which are common misconceptions or just dumb mistakes caused by the tendency of many to read variable names instead of documentation. There are not many of these conditions in our simple code base which makes it undesirable to design a normal error scheme to support this use case. One idea is to provide a -DDEBUG enabled structure that can store this type of error data when enabled (e.g. sort of a posix errno scheme). Other ideas are welcome but the end result would be to make libudpard a bit more friendly to new users.