Ensure flask context for job_failure handlers

For the flask_spinach object, similarly to the existing job_started and
job_finished signal handlers, there's now also a job_failed handler that
subclasses can use. These handlers are in place to ensure that the app
context is pushed before the handlers are called.

spinach/contrib/flask_spinach.py

@@ -53,6 +53,14 @@ def job_finished(*args, job=None, **kwargs):
                 # This means we didn't create the context. Ignore.
                 pass
 
+        @signals.job_failed.connect_via(namespace)
+        def job_failed(args, job=None, **kwargs):
+            if not flask.has_app_context():
+                ctx = app.app_context()
+                ctx.push()
+                flask.g.spinach_ctx = ctx
+            self.job_failed(job)
+
     @classmethod
     def job_started(cls, *args, job=None, **kwargs):
         """Callback for subclasses to receive job_started signals.
@@ -73,6 +81,18 @@ def job_finished(cls, *args, job=None, **kwargs):
         """
         pass
 
+    @classmethod
+    def job_failed(cls, *args, job=None, **kwargs):
+        """Callback for subclasses to receive job_failed signals.
+
+        There's no guarantee of ordering for Signal's callbacks,
+        so use this callback instead to make sure the app context
+        was pushed. If the signal is called as part of dead broker
+        detection, you will need to use this as normal signals may
+        not have been called with the app context pushed.
+        """
+        pass
+
     @property
     def spin(self):
         if self.app is not None: