backend/remote: Unlock workspace if Context fails

Previously any error case in the Context method would cause us to leave
the remote workspace locked on exit, requiring manual action to unlock it.

backend/remote/backend_context.go

@@ -49,6 +49,17 @@ func (b *Remote) Context(op *backend.Operation) (*terraform.Context, statemgr.Fu
 		return nil, nil, diags
 	}
 
+	defer func() {
+		// If we're returning with errors, and thus not producing a valid
+		// context, we'll want to avoid leaving the remote workspace locked.
+		if diags.HasErrors() {
+			err := op.StateLocker.Unlock(nil)
+			if err != nil {
+				diags = diags.Append(errwrap.Wrapf("Error unlocking state: {{err}}", err))
+			}
+		}
+	}()
+
 	log.Printf("[TRACE] backend/remote: reading remote state for workspace %q", workspace)
 	if err := stateMgr.RefreshState(); err != nil {
 		diags = diags.Append(errwrap.Wrapf("Error loading state: {{err}}", err))