Add check that symlinks exist.

letsencrypt/storage.py

@@ -128,6 +128,15 @@ class RenewableCert(object):  # pylint: disable=too-many-instance-attributes
         self.fullchain = self.configuration["fullchain"]
 
         self._fix_symlinks()
+        self._check_symlinks()
+
+    def _check_symlinks(self):
+        def check(link):
+            return os.path.exists(os.path.realpath(link))
+        for kind in ALL_FOUR:
+            if not check(getattr(self, kind)):
+                raise errors.CertStorageError(
+                        "link: {0} does not exist".format(getattr(self, kind)))
 
     def _consistent(self):
         """Are the files associated with this lineage self-consistent?