Unable to load the mail status livetable due to syntax error in JSON parsing

In the administration pane reporting the status of email sent, the livetable may not load due to errors in the parsing of the returned JSON. This happen because the error summary message is not escaped properly.

We need to check that all value are properly escaped.
At the same time, it would be nice to fix the single quote used for strings, since the JSON standard only support double quoted one.
Probably, using a java object serialization in place of manually composing the JSON would help fixing all these issues.