Get search radius from API rather than plist.

We had been getting the radius for location and address searches from a
hard coded value it the plist. This was a problem in that the number was
too small for some instances. Better is to calculate the radius from the
bounding box. This was implemented in the API since most of the
geoprocessing is handled there anyway. As a result we only need to grab
the proper key and set it in the app.
Reset instance string if there is a server error.

The way we were handling errors caused us to get into an infinite loop
of rejection of there was a problem with the instance string and the
server responded with an error. This could happen if a user used the app
on an instance and that instance later expired. Reopening the app would
result in a "server error" loop that could only be solved by crashing
out of the app. This resets the instance string if such a failure
occurs. On the cloud app this will result in having to choose another
instance.