Code review comment for lp:~javier.collado/utah/bug1161855

A test case to make sure the fail_pattern works has been added and the
documentation has been updated.

Regarding testing, what I've done is test the pattern that is used to detect
problem in the late/success_command. However, I haven't been able to test the
ones for installation failures (either in ubiquity or in d-i). I could send a
message that matches those patterns to syslog, but that won't test that those
messages are really printed for a broken image (when I stumble upon one I'll
take a look at the syslog to check how the error message is written).