Handle error with combine (#16)

* check if failed when combine image

* add status code when combine failed

prod/upload/combine.lua

@@ -13,20 +13,24 @@ local origin = {}
 for i = 1, len do
     origin[i] = file_dir .. "/" .. data[i] .. " "
 end
--- in linux
+-- linux
 local magick = "/usr/local/ImageMagick/bin/magick"
 local tmp = io.open(magick,"r")
 if tmp == nil then
-    -- in mac os
+    -- mac os
     magick = "/usr/local/bin/magick"
 else
     io.close(tmp)
 end
-local file = io.popen(magick .. " convert -append " .. table.concat(origin, " ") .. " " .. combined)
-file:read("*all")
-file:close()
-file = io.open(combined)
+
+local result, _, code = os.execute(magick .. " convert -append " .. table.concat(origin, " ") .. " " .. combined)
+if result and code == 0 then
+    local file = io.open(combined)
     local content = file:read("*all")
     file:close()
     os.remove(combined)
     ngx.say(content)
+else
+    ngx.status = 500
+    ngx.say("image combine failed.")
+end