docs: print out invalid branch name

to help user to understand what the extension is expecting.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb/scylladb#19477

docs/_ext/scylladb_common_images.py

@@ -136,7 +136,8 @@ class BaseVersionsTemplateDirective(Directive):
         if current_version.startswith(prefix):
             version = current_version
         elif not stable_version.startswith(prefix):
-            LOGGER.error("Invalid stable_version format in conf.py. It should start with 'branch-'")
+            LOGGER.error("Invalid stable_version format '%s' in conf.py. It should start with 'branch-'",
+                         stable_version)
         else:
             version = stable_version