Fix exception on import in MacOS

This is related to my fix in #658. Apparently, MacOS adds a git
executable that is just a stub which displays an error. This gets past
the try/except I added in #658, and allows all of the GitPython
components to be imported, but since the executable is not *actually*
git, it results in an exception when ``refresh()`` attemepts to run a
``git version``.

Author: terminalmage

Diff for: git/__init__.py

@@ -79,5 +79,8 @@ def refresh(path=None):
 #} END initialize git executable path
 
 #################
-refresh()
+try:
+    refresh()
+except Exception as exc:
+    raise ImportError('Failed to initialize: {0}'.format(exc))
 #################