Informative errors on qiita_env

Fixes #140

I've also noticed that `qiita_env make_demo_env` now takes a while since it has to download the data files. Since this can be annoying to the user, I've added some `print` statements such that the user know what is the script doing. Here is an example:
```
$ qiita_env make_demo_env
Creating database
Building SQL layout
Initializing database
Populating database with demo data (1/2)
Downloading test files
Extracting files
Populating database with demo data (2/2)
Demo environment successfully created
```
I don't think the script should error, it should just print a message saying that there's nothing to do and hinting to `qiita_env drop_demo_env`.