Macbook Pro Yosemite l Bootcamp Issue
If the Windows Support Software from the Boot Camp Assistant is not downloading and/or returns an error, you can download the software manually for the machine depending on the year/make of your Macbook, iMac, or Mac Pro.

This list can be found on the Apple KBase.

Based on you having a Mid-2012 13" MacBook, you would be looking for one of the following downloads:

Windows 7/8 64-bit (BC Support Software v5)
Windows 7 32-bit (BC Support Software v4)

Follow the instructions on the KBase article for installing the support software, and build your Win7 disk with the Support Software box unchecked in BCA.