Delete channel or resources task fails when attempting to release an expired Redis lock

### Observed behavior
Both `Delete channel` and `Manage > Delete (all) resources` for that same channel are failing. 
Asset from `v0.14.4-weekly7` build on Windows 7.
The only content present was **KA (EN) > Math > Arithmetic**, and I was not able to delete neither the full channel, nor by selecting all the resources inside, as the task was reported as `Failed`. I suspect just the DB was deleted but not the resources.

### Expected behavior
Being able to delete channel/resources.

### User-facing consequences
Inability to manage channels and resources on the device.

### Errors and logs

Nothing jumped at me as relevant in the browser console, attaching the logs.

[logs.zip](https://github.com/learningequality/kolibri/files/5497727/logs.zip)


### Steps to reproduce
1. Download **KA (EN) > Math > Arithmetic**.
2. Try to delete the channel and/or all the resources.

### Context

Tell us about your environment, including:
 * Kolibri version: v0.14.4-weekly7
 * Operating system: Windows 7
 * Browser: Firefox


diff --git a/kolibri/core/utils/cache.py b/kolibri/core/utils/cache.py
index c23672e622..3194e97d1f 100644
--- a/kolibri/core/utils/cache.py
+++ b/kolibri/core/utils/cache.py
@@ -61,7 +61,12 @@ class ProcessLock(object):
         self._lock.acquire()
 
     def release(self):
-        self._lock.release()
+        try:
+            self._lock.release()
+        except AssertionError:
+            logger.warning(
+                "Got an AssertionError when releasing a lock! This is likely from the lock TTL expiring."
+            )
 
     def __enter__(self):
         self.acquire()
['kolibri/core/utils/cache.py']