Removing Envs is buggy

[da-h]

Bug Description

1. When removing an env in the "Environment Management Modal" visdom will crash if the env has not been saved explicitly.
2. Removing an active env results in a state where no env is selected, but all windows of the removed env are still visible.

As a side note: One could put the os.remove into a try/catch-block, but with the second issue above, it seems this should be caught before that. Possibly there is a flag missing somewhere in the call chain that specifies if the env actually exists as a file or only in memory.

Reproduction Steps
See #886: The test cypress/integration/modal.js contains two TODOs that reproduce those issues repeatable.

Expected behavior
All windows should be cleared when removing the active env, also the visdom client should not crash when closing any env.

Server logs:

INFO:tornado.access:200 POST /env/text_fork_561637 (127.0.0.1) 0.72ms
INFO:root:from web client: {"cmd":"delete_env","prev_eid":"text_fork_561637","eid":"text_fork_561637_fork"}
INFO:root:closing environment text_fork_561637_fork
INFO:root:from web client: {"cmd":"delete_env","prev_eid":"text_fork_561637","eid":"text_fork_561637"}
INFO:root:closing environment text_fork_561637
ERROR:tornado.application:Uncaught exception GET /socket (127.0.0.1)
HTTPServerRequest(protocol='http', host='localhost:8098', method='GET', uri='/socket', version='HTTP/1.1', remote_ip='127.0.0.1')
Traceback (most recent call last):
  File "/home/da-h/.pyenv/versions/3.8.13/lib/python3.8/site-packages/tornado/websocket.py", line 635, in _run_callback
    result = callback(*args, **kwargs)
  File "/home/da-h/Projects/visdom/py/visdom/server/handlers/socket_handlers.py", line 264, in on_message
    os.remove(p)
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/testdir/text_fork_561637.json'