FIX error message on import from source directory (#276)

This error occurs every time `formats` doesn't import from `cortex`. There can be other reasons for this than the stated one, for example that `formats.pyx` wasn't compiled correctly.

cortex/__init__.py

@@ -12,7 +12,9 @@
 try:
     from cortex import formats
 except ImportError:
-    raise ImportError("You are running pycortex from the source directory. Don't do that!")
+    raise ImportError("Either are running pycortex from the source directory, or the build is broken. "
+                      "If your current working directory is 'cortex', where pycortex is installed, then change this. "
+                      "If your current working directory is somewhere else, then you may have to rebuild pycortex.")
 
 load = Dataset.from_file