A blank block is displayed if the content json is malformed

Reproduction steps:

• Edit a document
• Insert a charjs macro
• Set a malformed json (e.g., use single quotes instead of double quotes for a string value)

Expected result:

• A message is displayed to the user, explaining that the json is malformed

Actual results:

• The macro displays a blank block, which is not very indicative that something wrong is going on

Note: a "SyntaxError: JSON.parse: unexpected character at line 5 column 20 of the JSON data" is printed in the console, but that not really user-friendly

Note: the affect version needs to be refined.