Show additional information for JIRA integration connection errors in the GUI

Problem

Currently, when setting up the native JIRA integration in GitLab and there is a connection problem, we just get the following error:

Connection failed. check your integration settings

In order to get more information on the error, the logs have to be reviewed. On top of that, if the user is a GitLab.com user, they will need support intervention as they don't have access to logs.

Some causes for this "Connection failed" error are certificate verification failures (self-signed cert or hostname mistmatch), or authentication/authorization issues between GitLab and JIRA.

Proposal

Surface the actual error code/message to the GUI so users can troubleshoot this more efficiently.