Detect common installation mistakes and show error message

[pokey]

• User forgets to clone community => Look for action we depend on and show error message during init if not detected
• User clones monorepo instead of cursorless-talon => Have python file at root of mono-repo that shows error message
• user hasn't installed talon?
• user hasn't installed cursorless-talon (note that we can look for .cursorless directory, tho old installations won't have it)
• user missing command-server

Note that if they just want cursorless keyboard, they don't need any of these things, so figure out how to handle

See #1951 for an example where a user tried to use cursorless without Talon community repo

[AndreasArvidsson]

Sounds good