Better error reporting in Crowbar webui

After adding the feature to release/pebbles/master, this is now the pull request to bring it to release/roxy/master

Better error reporting in Crowbar via the web interface to support the user:
The last logged lines of the respective chef client run are now displayed if
a propsal fails.

This change has been discussed in https://github.com/crowbar/barclamp-crowbar/pull/747 before.