Error message when loading an mbtiles file that isn't created from osgearth

[jasonbeverage]

When you load an earth file with an mbtiles layer with a non-osgearth layer you get a complaint like:

osgEarth]* JSON decoding error: * Line 1, Column 1
  Syntax error: value, object or array expected.

This might be from trying to load the profile and there not being one in the mbtiles.

[gwaldron]

We should prob assume it's spherical-mercator per the mbtiles spec?

[jasonbeverage]

Added a check to see if the profileStr might be JSON before attempting to parse it.