`by install` fails if `bt_mediator` is not in path.

[jamespharvey20]

I ran:

• pip install --user brotab
• ~/.local/bin/bt install

Traceback (most recent call last):
  File "/home/mdarling/.local/bin/bt", line 8, in <module>
    sys.exit(main())
  File "/home/mdarling/.local/lib/python3.8/site-packages/brotab/main.py", line 670, in main
    exit(run_commands(sys.argv[1:]))
  File "/home/mdarling/.local/lib/python3.8/site-packages/brotab/main.py", line 663, in run_commands
    result = args.func(args)
  File "/home/mdarling/.local/lib/python3.8/site-packages/brotab/main.py", line 352, in install_mediator
    manifest = template.replace(r'$PWD/brotab_mediator.py', bt_mediator_path)
TypeError: replace() argument 2 must be str, not None

This happens because of line 327:

    bt_mediator_path = shutil.which('bt_mediator')

Because I had not yet added ~/local/bin to my path, which returns nothing. Obviously, adding it to my path is the fix for me. But, it might be worth:

• Having bt detect if bt install has been ran, and erroring if it has not.
• Erroring if bt_mediator_path is empty, saying it must be in path.
• Mentioning adding to path in the readme installation section.