fix agreement URL updating

[aripringle]

The terms URL recently updated (to https://letsencrypt.org/documents/LE-SA-v1.1.1-August-1-2016.pdf), and it seems that the method of automatically updating this wasn't functioning.

I've changed it to use the error message in the response to get the new URL. This may not be the ideal approach, since I presume that localization could affect what the response text is, but it's a quick fix that worked for me.