Some reporters are aggregating the messages instead of displaying
them when they are available. The actual displaying was conflatted
in the generate_reports. Unfortunately this behaviour was flaky
and in the case of the JSON reporter, the messages weren't shown
at all if a file had syntax errors or if it was missing.
In order to fix this, the aggregated messages can now be
displayed with Reporter.display_message, while the reports are
displayed with display_reports.
Close#766 and #765
