问题情况：
运行.DbMigrator时出现报错
解决方法：
在 .EntityFrameworkCore项目终端运行——dotnet ef database drop 删除数据库，重新运行 .DbMigrator
BookStore.EntityFrameworkCore> dotnet ef database drop
Build started. . .
Build succeeded.
Are you sure you want to drop the database 'BookStore' on server'(LocalDb)/NSSQLLocalDB'? (y/N)
y
Dropping database 'BookStore' on server '(LocalDb) \MSsQLLocalDB'.successfully dropped database 'BookStore" .