server app must check http status code

[edublancas]

there are a bunch of places where we're making requests but we're not checking that the response code is 200. we should do it and then show an error if the code is != 200

jupysql-plugin/jupysql_plugin/server_handlers/dashboard.py

Line 103 in a5986a0

res = make_request(headers=headers, files=upload_files)

we should also add a ui test to ensure that the response error is shown if the code is != 200