Adds ErrWorkspaceLockedStateVersionStillPending#978

brandonc merged 2 commits intomainhashicorp/go-tfe:mainfrom

TF-20458-improve-hcp-terraform-workspace-unlock-retry-messaging-in-terraform-clihashicorp/go-tfe:TF-20458-improve-hcp-terraform-workspace-unlock-retry-messaging-in-terraform-cliCopy head branch name to clipboard