Mark error handling for group indexing complete.
Mark error handling for group indexing complete.

Could still improve what the NSError instance returns, but it doesn't completely blow up on index out of range errors now.
rnewman/SWXMLHash,yas375/SWXMLHash,st3fan/SWXMLHash,chrisamanse/SWXMLHash,skipallmighty/SWXMLHash,st3fan/SWXMLHash,rnewman/SWXMLHash,rnewman/SWXMLHash,jpsim/SWXMLHash,yas375/SWXMLHash,fluffyemily/SWXMLHash,st3fan/SWXMLHash,fluffyemily/SWXMLHash,chrisamanse/SWXMLHash,chrisamanse/SWXMLHash,yas375/SWXMLHash,drmohundro/SWXMLHash,skipallmighty/SWXMLHash,skipallmighty/SWXMLHash,st3fan/SWXMLHash,drmohundro/SWXMLHash,fluffyemily/SWXMLHash,jpsim/SWXMLHash,jpsim/SWXMLHash