health check updates

[yesoreyeram]

• currently health check retrieves list of all repos within grafana org. This requires lots of pagination and slow. Instead this PR filters the number of repos to github-datasource. with this no changes in existing behaviour but this will bit faster.
• also when invalid/empty credentials entered, we currently show "health check failed". Instead this PR will show little more context. ( underlying library abstracts the error https://github.com/shurcooL/graphql/blob/ed46e5a4646634fc16cb07c3b8db389542cc8847/graphql.go#L79 . also we don't want to show this error message as retrieved for security reasons )