Check system date when starting packaged Marketplace (bug 1019122)

If the device clock is too far behind the user will get a certificate error, so let's display our own error message instead.

~~WIP, need translations first, the translations for the error messages generated by yulelog package are directly hardcoded into the package main.js file :(~~
So, I used `dateerror` on purpose, to differentiate the class I used on the body from the class used on the `<div>` that holds the error message. And to make it similar to the `offline` class that's also applied to the `body`. I could change it if you prefer it that way though.