Add command output on failed commands

Change-Id: I3b7e98cca90bc2ee96ba5eb4bb1c69050e70ed3c
Reviewed-on: https://gerrit.instructure.com/102634
Tested-by: Jenkins
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Brad Horrocks <bhorrocks@instructure.com>

lib/brandable_css.rb

@@ -263,9 +263,10 @@ module BrandableCSS
 
       percent_complete = 0
       Open3.popen2e(command) do |_stdin, stdout_and_stderr, wait_thr|
+        error_output = []
         stdout_and_stderr.each do |line|
           Rails.logger.try(:debug, line.chomp!) if defined?(Rails)
-
+          error_output.push(line)
           # This is a good-enough-for-now approximation to show the progress
           # bar in the UI.  Since we don't know exactly how many files there are,
           # it will progress towards 100% but never quite hit it until it is complete.
@@ -276,7 +277,10 @@ module BrandableCSS
             opts[:on_progress].call(percent_complete)
           end
         end
-        raise("Error #{msg}") unless wait_thr.value.success?
+        unless wait_thr.value.success?
+          STDERR.puts error_output.join("\n")
+          raise("Error #{msg}")
+        end
       end
     end
   end