Fix unintialized constant exception handler error

When `gitlab-ctl reconfigure` hits an HTTP error while trying to
download local cookbooks, the application runs into an `uninitialized
constant GitLabHandler::Exception::OmnibusHelper` error:

```
ERROR: Running exception handlers
ERROR: Report handler GitLabHandler::Exception raised #<NameError: uninitialized constant GitLabHandler::Exception::OmnibusHelper>
ERROR: /opt/gitlab/embedded/cookbooks/package/libraries/handlers/gitlab.rb:30:in `report'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/handler.rb:296:in `run_report_unsafe'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/handler.rb:284:in `run_report_safely'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/handler.rb:155:in `block in run_exception_handlers'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/handler.rb:153:in `each'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/handler.rb:153:in `run_exception_handlers'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/handler.rb:164:in `block in <class:Handler>'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/client.rb:443:in `block in run_failed'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/client.rb:442:in `each'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/client.rb:442:in `run_failed'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/client.rb:306:in `rescue in run'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/client.rb:299:in `run'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/application.rb:305:in `run_with_graceful_exit_option'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/application.rb:281:in `block in run_chef_client'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/local_mode.rb:42:in `with_server_connectivity'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/application.rb:264:in `run_chef_client'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/application/base.rb:352:in `run_application'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-17.10.0/lib/chef/application.rb:67:in `run'
ERROR: /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/chef-bin-17.10.0/bin/cinc-client:25:in `<top (required)>'
ERROR: /opt/gitlab/embedded/bin/cinc-client:25:in `load'
ERROR: /opt/gitlab/embedded/bin/cinc-client:25:in `<main>'
ERROR: Exception handlers complete
```

This was seen in
https://gitlab.com/gitlab-com/gl-infra/production/-/issues/16897 and
raised as a side issue in
https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/8245.

Changelog: fixed

Author: stanhu

files/gitlab-cookbooks/package/libraries/handlers/gitlab.rb

@@ -16,6 +16,7 @@
 
 require 'chef/handler'
 require 'rainbow'
+require_relative '../omnibus_helper'
 
 module GitLabHandler
   class Exception < Chef::Handler