store: Return an error from GetBlueprintChange

If the blueprint doesn't exist, or the commit for the selected blueprint
doesn't exist it will return an error.

This also fixes the blueprints/undo/ route to return the correct error
to the caller.

internal/store/store.go

@@ -354,15 +354,20 @@ func (s *Store) GetBlueprintCommitted(name string) *blueprint.Blueprint {
 	return &bp
 }
 
-func (s *Store) GetBlueprintChange(name string, commit string) *blueprint.Change {
+// GetBlueprintChange returns a specific change to a blueprint
+// If the blueprint or change do not exist then an error is returned
+func (s *Store) GetBlueprintChange(name string, commit string) (*blueprint.Change, error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
+	if _, ok := s.BlueprintsChanges[name]; !ok {
+		return nil, errors.New("Unknown blueprint")
+	}
 	change, ok := s.BlueprintsChanges[name][commit]
 	if !ok {
-		return nil
+		return nil, errors.New("Unknown commit")
 	}
-	return &change
+	return &change, nil
 }
 
 // GetBlueprintChanges returns the list of changes, oldest first

internal/weldr/api.go

@@ -1249,15 +1249,26 @@ func (api *API) blueprintUndoHandler(writer http.ResponseWriter, request *http.R
 
 	name := params.ByName("blueprint")
 	commit := params.ByName("commit")
-	bpChange := api.store.GetBlueprintChange(name, commit)
+	bpChange, err := api.store.GetBlueprintChange(name, commit)
+	if err != nil {
+		errors := responseError{
+			ID:  "BlueprintsError",
+			Msg: err.Error(),
+		}
+		statusResponseError(writer, http.StatusBadRequest, errors)
+		return
+	}
+
 	bp := bpChange.Blueprint
 	commitMsg := name + ".toml reverted to commit " + commit
-	err := api.store.PushBlueprint(bp, commitMsg)
+	err = api.store.PushBlueprint(bp, commitMsg)
 	if err != nil {
-		statusResponseError(writer, http.StatusInternalServerError, responseError{
-			ID:  "BlueprintError",
+		errors := responseError{
+			ID:  "BlueprintsError",
 			Msg: err.Error(),
-		})
+		}
+		statusResponseError(writer, http.StatusBadRequest, errors)
+		return
 	}
 	statusResponseOK(writer)
 }