Use Project#get_by_name instead of #find_by in Staging

Staging::StagingProject is a subclass of Project. `get_by_name` should
be used always that a project is required instead of `find_by!` as it
considers more cases where an exception should be raised.

src/api/app/controllers/staging/staged_requests_controller.rb

@@ -62,7 +62,7 @@ def request_numbers
   end
 
   def set_staging_project
-    @staging_project = Staging::StagingProject.find_by!(name: params[:staging_project_name])
+    @staging_project = Staging::StagingProject.get_by_name(params[:staging_project_name])
   end
 
   def set_staging_workflow

src/api/app/controllers/staging/staging_projects_controller.rb

@@ -2,6 +2,6 @@ class Staging::StagingProjectsController < ApplicationController
   skip_before_action :require_login
 
   def show
-    @staging_project = Staging::StagingProject.find_by!(name: params[:name])
+    @staging_project = Staging::StagingProject.get_by_name(params[:name])
   end
 end