The "Fetcher failure for URL: ..." error is due to the 3.11 branch being actually named "socfpga-3.11", without the trailing "-rel".
To solve it I edited the 2nd line of file "~/altera-yocto/meta-altera/recipes-kernel/linux/linux-altera.inc" from:
SRC_URI = "git://
git.rocketboards.org/linux-socfpga.git;protocol=http;branch=socfpga-${PV}-rel"
to
SRC_URI = "git://
git.rocketboards.org/linux-socfpga.git;protocol=http;branch=socfpga-${PV}"
Regarding the second error "Multiple .bb files ..." I came here looking for a solution to that :) I can't get a dtb either!