If you are seeing the error “No accessible source location found for the content” in execmgr.log on your ConfigMgr 2012 clients just after an entry such as “Executing program setup.exe” you have configured your package and deployment in a way this makes it impossible for the package to install.
It seems strange to me that the product will even allow this to happen however if you select “Run program from distribution point” on the deployment.
And you did not select “copy the content in this package to a package share on distribution points” it will always fail to install. Be aware though, checking “copy the content in this package to a package share on distribution points” doubles the amount of disk space required on your distribution point. It also provides a fallback location for clients to get content if they cannot use BITS.