Better error message when up.json is missing

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [x] I inspected the verbose debug output with the `-v, --verbose` flag.
* [ ] Are you an Up Pro subscriber?

## Description

I came across an error message which could possibly be improved, while I was trying to trying to set up CI on my project. My project is a mono repo, and the `up` project is in a subdirectory. I was being daft, and running up in the root directory, instead of the subdirectory with `up.json`. It took me quite a while to find out what the problem was, because of the error message.

## Steps to Reproduce

```sh
docker run -ti node:6.10 /bin/bash
# inside the docker container
curl -f https://up.apex.sh/install | sh
mkdir ./empty-dir
cd ./empty-dir
up
```

```sh
# error message:
Error: Cannot find credentials, visit https://up.docs.apex.sh/#aws_credentials for help.
```

```sh
# expected something like
No up.json found ...
```

Note, if I run `up` [`0.4.10`] on my OS X 10.12.6, I get the expected:

```
No up.json found, create a new project? (Y/n)
```