that means the stock recovery is rejecting the update.zip. Take the recovery image that I'm about to give you, then place it in the same directory as your fastboot. After this, boot into fastboot by either holding volume down + power or the back button + power, then press the power button again. Then go to your computer, and on the terminal or command prompt, type cd [the directory where fastboot is] and type fastboot flash recovery recovery.img! If it successfully flashes the recovery(which it should) then your recovery will be Clockworkmod.
Starting as a dev is hard, :P.
Always remember: Do what you love, continue to do it, and it will take you where you need to go.
Device: HTC Sensation 4G
Rom: ViperS 1.2.1
Kernel: Bricked 3.0 1.4
SD Card: 32 GB Class 10
Backup Device 1(If it gets fixed): T-Mobile myTouch 3G Slide
Rom: Constantly changing.
Root Method: Unlocked Bootloader
Recovery: ClockworkMod 5
Kernel: DrowningChild Kernel With XTra OC