tfdiags: show descriptions in diagnosticsAsError
tfdiags: show descriptions in diagnosticsAsError

Previously we were showing only the summaries when converting to a string
error, but HCL generates summaries that indicate only the _type_ of error,
expecting that the detail will give the details, and so we need to show
both in order to produce a useful error message.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk