Better exception handling

Better exception handling

Instead of catching all exceptions that can occurs and miss one, let's
create a "parent" exception class for OnapTest and catch only this one.

In order to know what's the error, we also create a class attribute
(`error_message`) that gives us the reason for exception.

Issue-ID: INT-1796
Signed-off-by: Sylvain Desbureaux <sylvain.desbureaux@orange.com>
Change-Id: I798d8c6270c466b1d9be6511f52fd9441401c9c8
(cherry picked from commit 9c71e24adaaedd07d7e2e65922c5a3cee27318c9)