Add cross reference handling, improve error handling

The changes in my fork do the following:

- abort if there's an error getting the existing issues.  Failing to do so can lead to the creation of duplicates.
- changed existing issues has to use the original Google Code issue number instead of the title to deal with a) duplicate titles and b) titles that change of the GC side NOTE: this may not be backward compatible if the link hasn't always been included or the format of it changed !!
- escaped # signs so they don't get interpreted as issue references
- convert Google Code style Issue nnn xrefs to Github style xrefs NOTE: this is incomplete! It works for a clean repo where there's a one to one mapping between issue numbers, but needs extension to support non 1:1 mappings.