maktaba#plugin#Install('nonexistent/path/') should throw an error

[dbarnett]

Calling maktaba#plugin#Install('nonexistent/path/') to install nonexistent plugin paths actually succeeds, registering and returning a plugin object:

:echo maktaba#plugin#Install('nonexistent/path/')
{'HasDir': function('maktaba#plugin#HasDir'), 'flags': {}, …}
:echo maktaba#plugin#IsRegistered('path')
1

Seems like it should throw NotFound or something.