Set status as "Error" if downloaded zip is invalid

(#29)

src/util.cr

@@ -81,3 +81,15 @@ end
 def compare_alphanumerically(a : String, b : String)
   compare_alphanumerically split_by_alphanumeric(a), split_by_alphanumeric(b)
 end
+
+# When downloading from MangaDex, the zip/cbz file would not be valid
+#   before the download is completed. If we scan the zip file,
+#   Entry.new would throw, so we use this method to check before
+#   constructing Entry
+def validate_zip(path : String) : Exception?
+  file = Zip::File.new path
+  file.close
+  return
+rescue e
+  e
+end