`func host start` allows running in invalid directory

If you run `func host start` in a directory outside of the project, it exits with the error: `Unable to find project root.`. This is good behavior.

However, if you run `func host start` in a function subdirectory (e.g. `HttpTrigger` from the examples), then it will create a new `host.json` file and start. This is inconsistent behavior. A preferred behavior would be to exit similar to if it was outside of the directory.

Reproduce `func host start` from a function subdirectory such as `HttpTrigger`, then trace the project-root validation and `host.json` creation path. Done means the command exits like it does outside the project and does not create a new `host.json` file.

Written by the indexing model from the issue text.