add test for process error codes

features/step_definitions/errors.rb

@@ -0,0 +1,15 @@
+
+When /^I preprocess data$/ do
+  begin
+    osrm_kill
+    reprocess
+  rescue OSRMError => e
+    @process_error = e
+  end
+end
+
+Then /^preparing should return code (\d+)$/ do |code|
+  @process_error.class.should == OSRMError
+  @process_error.process.should == 'osrm-prepare'
+  @process_error.code.to_i.should == code.to_i
+end