github/config: Fix detection of individual, non-org accounts (#685)

- Previously, whenever an individual user tried to interact with their
  repos, the provider would return an error, rendering v4.3.1 unusable
  _for individuals_:

  ```
  ➜ terraform plan
  Error: GET https://api.github.com/orgs/issyl0: 404 Not Found []
  on /Users/issyl0/repos/terraform/github.tf line 1, in provider "github":
  ```

- `ConfigureOwner` works such that if the `owner.name` is not blank (ie,
  the user had specified `owner = <username>` in their Terraform file),
  the code progresses to check if the `owner.name` is an org.
  Importantly, that check (prior to this change) returned an error if
  `owner.name` was not an org. That final error meant it was impossible
  to run if not using an organisation account.

- Reproduction steps: https://gist.github.com/issyl0/cd61e4cb59de2c2e1e8f45e3cf7c12f5

Author: issyl0

github/config.go

@@ -115,8 +115,6 @@ func (c *Config) ConfigureOwner(owner *Owner) (*Owner, error) {
 				owner.id = remoteOrg.GetID()
 				owner.IsOrganization = true
 			}
-		} else {
-			return nil, err
 		}
 	}