don't treat dot's output into stderr as error. check dot's return code

[375gnu]

In my Debian Sid I've got the following:

/usr/lib/ruby/vendor_ruby/graphviz/utils.rb:76:in `output_from_command': Error from "/usr/bin/dot" -q1   -Tpng -ohello_world.png   /tmp/graphviz.rb20130629-29282-1uhwdt2: (RuntimeError)
Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 9: reading configurations from ~/.fonts.conf is deprecated.
  from /usr/lib/ruby/vendor_ruby/graphviz.rb:655:in `output'
  from tst.rb:15:in `<main>'

As you can see there is warning from fontconfig which you treat as dot's error. Please, don't do that, check return code first.

[375gnu]

Possible patch which uses Open3.capture3 may look like

--- utils.rb.orig   2013-06-29 21:38:23.000000000 +0300
+++ utils.rb    2013-06-29 21:47:41.000000000 +0300
@@ -55,11 +55,8 @@
      end
    end
    begin
-     Open3.popen3( cmd ) do |stdin, stdout, stderr|
-       stdin.close
-       stdout.binmode
-       [stdout.read, stderr.read]
-     end
+     outstr, errstr, status = Open3.capture3( cmd )
+     [outstr, errstr, status.exitstatus]
    rescue NotImplementedError, NoMethodError
      IO.popen( cmd ) do |stdout|
        stdout.binmode
@@ -69,8 +66,8 @@
   end

   def output_from_command(cmd) #:nodoc:
-   output, errors = output_and_errors_from_command(cmd)
-   if errors.nil? || errors.strip.empty?
+   output, errors, exitstatus = output_and_errors_from_command(cmd)
+   if exitstatus.zero?
      output
    else
      raise "Error from #{cmd}:\n#{errors}"

[glejeune]

done