wp & modules: make wp.fetch() available to others

Hi Tridge,

As we discussed this change makes wpt fetch() available to other modules.  I also included a small change to mavproxy.py to show the stack trace when a cmd throws an exception (eases module debugging).  My api stuff uses this fetch() method.

(Just rebased to master but I'm happy to split into separate PRs or whatever makes it easier.  I can also add a bump to 1.31 if you'd prefer)