Catch network errors when testing integrations

Summary

With certain settings, testing an integration can result in a 500 error. The user sees a message like this:

Steps to reproduce

For example, using the Webex Teams integration and a URL of http://example.com:1234 results in a Net::OpenTimeout error:

• https://sentry.gitlab.net/gitlab/gitlabcom/issues/2505499/
• https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/2504924/

What is the current bug behavior?

An uncaught exception is raised.

What is the expected correct behavior?

The user should see an error message with more details.

Output of checks

This bug happens on GitLab.com

Possible fixes

One option would be to catch all *Gitlab::HTTP::HTTP_ERRORS at https://gitlab.com/gitlab-org/gitlab/blob/52ee44608ab9213674422e86ae462fa22420c47b/app/controllers/projects/services_controller.rb#L76, rather than just Gitlab::HTTP::BlockedUrlError.

The messages from these exceptions should be safe to pass on to the user.