Fix issue when generating test session report where exception['message_lines'] is a String

Fixes issue seen in https://ops.gitlab.net/gitlab-org/quality/canary/-/jobs/3234894

Exception's message_lines can also be a String such as in this example:

{"class"=>"QA::Resource::ApiFabricator::ResourceFabricationFailedError", "message"=>"Fabrication of QA::Resource::ProjectSnippet using the API failed (400) with `{\"message\":{\"error\":\"Repository Error creating the snippet\"}}`.", "message_lines"=>"Shared Example Group: \"copying snippet file contents\" called from ./qa/specs/features/browser_ui/3_create/snippet/copy_snippet_file_contents_spec.rb:77", "backtrace"=>[]}