Capture Mako exit code and fail fast if necessary

Capture the exit code of the Mako template engine invocation on
asset preprocessing and abort from the Rake task on failure. This
will prevent the LMS from continuing its attempt to start up,
preventing further configuration errors.

rakefiles/assets.rake

@@ -19,9 +19,11 @@ def preprocess_with_mako(filename)
     # strip off the .mako extension
     output_filename = filename.chomp(File.extname(filename))
 
-    # just pipe from stdout into the new file
+    # just pipe from stdout into the new file, exiting on failure
     File.open(output_filename, 'w') do |file|
       file.write(`python -c '#{mako}'`)
+      exit_code = $?.to_i
+      abort "#{mako} failed with #{exit_code}" if exit_code.to_i != 0
     end
 end