files cache: improve exception handling, fixes #3553

now deals with:
- corrupted files cache (truncated or modified not by borg)
- inaccessible/unreadable files cache
- missing files cache

The latter fix is not sufficient, the cache transaction processing
would still stumble over expected, but missing files in the cache.

src/borg/cache.py

@@ -502,7 +502,8 @@ def _read_files(self):
         self.files = {}
         self._newest_cmtime = None
         logger.debug('Reading files cache ...')
-
+        msg = None
+        try:
             with IntegrityCheckedFile(path=os.path.join(self.path, 'files'), write=False,
                                       integrity_data=self.cache_config.integrity.get('files')) as fd:
                 u = msgpack.Unpacker(use_list=True)
@@ -517,10 +518,16 @@ def _read_files(self):
                             # in the end, this takes about 240 Bytes per file
                             self.files[path_hash] = msgpack.packb(entry._replace(age=entry.age + 1))
                     except (TypeError, ValueError) as exc:
-                    logger.warning('The files cache seems corrupt, ignoring it. '
-                                   'Expect lower performance. [%s]' % str(exc))
+                        msg = "The files cache seems invalid. [%s]" % str(exc)
+                        break
+        except OSError as exc:
+            msg = "The files cache can't be read. [%s]" % str(exc)
+        except FileIntegrityError as fie:
+            msg = "The files cache is corrupted. [%s]" % str(fie)
+        if msg is not None:
+            logger.warning(msg)
+            logger.warning('Continuing without files cache - expect lower performance.')
             self.files = {}
-                    return
 
     def begin_txn(self):
         # Initialize transaction snapshot