SBPillar should give a more informative error if a file isn't accessable on the file exchange url given

Currently clients gets a checksum error if they try to put a file which isn't available on the file exchange server (not uploaded or url is wrong). This is very confusing, so the pillar should instead send a failure description informing what actually occurred.

Solution proposal: Check the file is accessable before starting the actual download. See this page for inspiration.