tiptorrent: check cache availability in tip_write_csum

Remove old TODO message.
Check if the cache is available before trying to generate an
image's checksum.

src/utils/tiptorrent.py

@@ -14,6 +14,7 @@ import shutil
 import subprocess
 import urllib.request
 from src.log import OgError
+from src.utils.cache import mount_cache
 
 def _compute_md5(path, bs=2**20):
     m = hashlib.md5()
@@ -41,9 +42,9 @@ def tip_fetch_csum(tip_addr, image_name):
 
 
 def tip_write_csum(image_name):
-    """
-    TODO: Check for CACHE partition
-    """
+    if not mount_cache():
+        raise OgError(f'Failed to checksum {image_name}: cache partition is not available')
+
     image_path = f'/opt/opengnsys/cache/opt/opengnsys/images/{image_name}.img'
 
     if not os.path.exists(image_path):