Provide better debug output if a pyximport fails.

pyximport/pyximport.py

@@ -217,7 +217,8 @@ def load_module(name, pyxfilename, pyxbuild_dir=None, is_package=False,
         if is_package and not hasattr(mod, '__path__'):
             mod.__path__ = [os.path.dirname(so_path)]
         assert mod.__file__ == so_path, (mod.__file__, so_path)
-    except Exception:
+    except Exception as failure_exc:
+        _debug("Failed to load extension module: %r" % failure_exc)
         if pyxargs.load_py_module_on_import_failure and pyxfilename.endswith('.py'):
             # try to fall back to normal import
             mod = imp.load_source(name, pyxfilename)