Upgrades, fix error message formatting

In prepare_upgrade_files function was
wrong strings concatination. As result
user saw bash errors when he run the
script.

Closes-bug: #1337937

blueprint upgrade-to-5-0-1

Change-Id: Iaf734aac537f07c10811a382cbe90c2fe7f8f393
Merge "Upgrades, fix error message formatting" into stable/5.0