Display a friendlier error message when DB is not seeded

What does this MR do and why?

Goes from undefined method personal_access_tokens' for nil:NilClass` as seen in gitlab-org/quality/engineering-productivity/review-apps-broken-incidents#30 (closed) to a friendlier message:

Error: Could not find root user. Check that the database was properly seeded

The first commit is a pure refactoring (i.e. we don't change the ruby command - see the validation in the next section).

The second commit adds the friendlier error message.

Does it work?

The review-deploy job for this MR succeeded: https://gitlab.com/gitlab-org/gitlab/-/jobs/4212705710#L185

How to set up and validate locally

In the first commit, I just extracted the one-liner ruby code to a heredoc. Below is to prove that the output before/after refactoring is the same (I added an extra ; at the end of the last line for consistency):

export REVIEW_APPS_ROOT_TOKEN=abcd

read -r -d '' multiline_ruby_code <<RUBY
token = User.find_by_username('root').personal_access_tokens.create(scopes: [:api], name: 'Token to disable sign-ups');
token.set_token('${REVIEW_APPS_ROOT_TOKEN}');
begin;
token.save!;
rescue(ActiveRecord::RecordNotUnique);
end;
Gitlab::CurrentSettings.current_application_settings.update!(signup_enabled: false);
RUBY

ruby_code=$(echo $multiline_ruby_code | tr '\n' ' ')
disable_signup_rb="token = User.find_by_username('root').personal_access_tokens.create(scopes: [:api], name: 'Token to disable sign-ups'); token.set_token('${REVIEW_APPS_ROOT_TOKEN}'); begin; token.save!; rescue(ActiveRecord::RecordNotUnique); end; Gitlab::CurrentSettings.current_application_settings.update!(signup_enabled: false)"

echo $ruby_code
# token = User.find_by_username('root').personal_access_tokens.create(scopes: [:api], name: 'Token to disable sign-ups'); token.set_token('abcd'); begin; token.save!; rescue(ActiveRecord::RecordNotUnique); end; Gitlab::CurrentSettings.current_application_settings.update!(signup_enabled: false); 
echo $disable_signup_rb
# token = User.find_by_username('root').personal_access_tokens.create(scopes: [:api], name: 'Token to disable sign-ups'); token.set_token('abcd'); begin; token.save!; rescue(ActiveRecord::RecordNotUnique); end; Gitlab::CurrentSettings.current_application_settings.update!(signup_enabled: false)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.