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

cython/gtsam/__init__.py

@@ -1,19 +1,26 @@
 from .gtsam import *
 
-import gtsam_unstable
+try:
+    import gtsam_unstable
 
 
-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))
-        return item(*args, **kwargs)
-    return wrapper
+    def _deprecated_wrapper(item, name):
+        def wrapper(*args, **kwargs):
+            from warnings import warn
+            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)
-        globals()[name] = item
+
+    for name in dir(gtsam_unstable):
+        if not name.startswith('__'):
+            item = getattr(gtsam_unstable, name)
+            if callable(item):
+                item = _deprecated_wrapper(item, name)
+            globals()[name] = item
+
+except ImportError:
+    pass