Only use pygit2.errors if it exists (2015.5 branch)

pygit2 0.20.3 is the only supported pygit2 for gitfs which doesn't have
this errors submodule. We need to account for it not being there.

Fixes #26245 for the 2015.5 branch

salt/fileserver/gitfs.py

@@ -115,6 +115,10 @@ except ImportError:
 try:
     import pygit2
     HAS_PYGIT2 = True
+    try:
+        GitError = pygit2.errors.GitError
+    except AttributeError:
+        GitError = Exception
 except ImportError:
     HAS_PYGIT2 = False
 
@@ -1159,7 +1163,7 @@ def update():
                     pass
                 try:
                     fetch = origin.fetch()
-                except pygit2.errors.GitError as exc:
+                except GitError as exc:
                     # Using exc.__str__() here to avoid deprecation warning
                     # when referencing exc.message
                     if 'unsupported url protocol' in exc.__str__().lower() \