[Buildbot] #3437: format configuration file syntax errors in more friendly way

[Buildbot trac]

#3437: format configuration file syntax errors in more friendly way
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Reporter:  rutsky     |       Owner:
    Type:  undecided  |      Status:  new
Priority:  major      |   Milestone:  undecided
 Version:  0.9.0b6    |  Resolution:
Keywords:             |
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Comment (by adityadivekar):

 According to the code, all non-config errors are added to the variable
 {{{_error}}} and others are included in the traceback as desired, if I'm
 not wrong.

 So should I make only the changes that you have highlighted in the ticket
 and commit them or are there any other features that I should include in
 the commit?

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Ticket URL: <http://trac.buildbot.net/ticket/3437#comment:4>
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