Document the exception when the target map file is not loaded.

If the targets map file cannot be loaded using load_json_file,
a tuf.exceptions.Error exception will be raised.

Signed-off-by: marinamoore <mnm678@gmail.com>

tuf/client/updater.py

@@ -693,6 +693,9 @@ def __init__(self, repository_name, repository_mirrors,
         If there is an error with the updater's repository files, such
         as a missing 'root.json' file.
 
+    tuf.exceptions.Error:
+      If the targets map file cannot be loaded.
+
     <Side Effects>
       Th metadata files (e.g., 'root.json', 'targets.json') for the top- level
       roles are read from disk and stored in dictionaries.  In addition, the