Do not run tasks if required mounts are not present.

A configuration item `imports.mounts` is added to
`etc/config.yaml`. This should be a list of paths
which must be non-empty. If any of the paths in that
list is empty, runner.sh will abort.

Closes #200.

etc/config.yaml

@@ -32,6 +32,13 @@ imports:
     # least this many seconds ago.
     file_min_age: 60
 
+    # These paths refer to remote mounts which must be present in order
+    # for imports to work. If any of these paths are empty, import actions
+    # (including data deletion) will be inhibited. This is to cope with
+    # things like transient network failures.
+    mounts:
+        - /srv/mnt/Data
+
 queues:
     asaqc:
         request: