If you had a second rom running on the safe side via safe strap, that is why you are failing the update. Safestrap places all of the files for your safe Rom in a folder called "preinstall" and the stock files that are supposed to be there are removed. When the ota update package checks your system it sees that your pre install folder is not right so the update fails.
You have two options. You can fastboot with rsd back to 744, then update to 748, root, use root keeper, then take ota for 173.
The other option is to go to droidrzr.com. they have a thread with a link to the files that should be in your pre install folder. You can download that to your computer, unzip, then delete whatever files are in your preinstall folder and place the correct ones in there.
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