First of all...do you get the boot option when booting your phone to boot into the Safestrap recovery? If not, it is not properly installed on your phone and is not the cause of the update not installing. Just having the Safestrap app on your phone does not mean Safestrap is active with an active recovery installed.
However...if it is actively installed then you need to follow the above steps to properly remove it before unrooting.
Your error saying you do not have SuperUser on the phone can be corrected if the phone is properly rooted by downloading SuperUser from the play store. SU is required in order for terminal to let you into SU commands.
Once the above commands are run from terminal (successfully) You can go into Safestrap and verify that recovery is nolonger installed...or chose the option to remove recovery if it is still installed. Then go into settings...apps and uninstall safestrap application.
I tried to install the update and got a FAIL this morning. Followed the above steps and then the update installed just fine. Now in process of re-rooting. Probably won't install safestrap again though. Just want root access for root explorer and debloating. Don't really care for the custom roms out today.