Better error reporting in Crowbar webui

Rewrite after feedback

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.
(Internal reference: https://bugzilla.novell.com/show_bug.cgi?id=797059)
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Added error handling for the get_log_lines method

Include Chef error report into log excerpt