DB access denied error improvement

[smallinsky]

When a user configures teleport the first time the default roles don't provide any db_users, and db_labels that allow a user to connect to db instance. During the db access flow teleport returns the access to db denied error that is omitted, or misunderstood by a teleport user.

tsh db connect eks-app-db
psql: error: connection to server at "localhost" (127.0.0.1), port 61378 failed: access to db denied`

TSH flow should return an actionable error for example instead of failed: access to db denied the error should indicate that a user is not assigned to any roles that provide valid db_users. Additionally, the tsh cli can check if a user has access to a particular database and return a detailed error. The work done is #10458 can be reused to determine if a user has access to a particular DB from tsh db login/connect flow.