[FIX] Report: don't shallow exceptions in report_download
[FIX] Report: html escape the error json string

session.get_file appends the json to the body of the generated iframe and
then tries to json.parse it by reading contentNode on the body.
Exceptions from `report_download` method may contain `<` and `>`, so when
json.parse tries to json.parse the contentNode, it reads only a part of
the original json string. htmlescaping the json string solves the issue
by preventing the content of the json string to be interpreted as html.