fixed __init__.py to not crash if gtsam_unstable doesn't exist

cython/gtsam/__init__.py

@@ -1,19 +1,26 @@
 from .gtsam import *
 
+try:
     import gtsam_unstable
 
 
-def deprecated_wrapper(item, name):
+    def _deprecated_wrapper(item, name):
         def wrapper(*args, **kwargs):
             from warnings import warn
-        warn('importing the unstable item "{}" from gtsam is deprecated. Please import it from gtsam_unstable.'.format(name))
+            message = ('importing the unstable item "{}" directly from gtsam is deprecated. '.format(name) +
+                       'Please import it from gtsam_unstable.')
+            warn(message, category=DeprecationWarning)
             return item(*args, **kwargs)
         return wrapper
 
+
     for name in dir(gtsam_unstable):
         if not name.startswith('__'):
             item = getattr(gtsam_unstable, name)
             if callable(item):
-            item = deprecated_wrapper(item, name)
+                item = _deprecated_wrapper(item, name)
             globals()[name] = item
 
+except ImportError:
+    pass
+