fix(refs): don't raise StopIteration

Fixes gitpython-developers#394

Author: Byron

.gitignore

@@ -1,6 +1,7 @@
 *.py[co]
 *.swp
 *~
+.venv/
 /*.egg-info
 /lib/GitPython.egg-info
 cover/

git/refs/symbolic.py

@@ -108,7 +108,7 @@ def _iter_packed_refs(cls, repo):
                     yield tuple(line.split(' ', 1))
                 # END for each line
         except (OSError, IOError):
-            raise StopIteration
+            return
         # END no packed-refs file handling
         # NOTE: Had try-finally block around here to close the fp,
         # but some python version woudn't allow yields within that.