conda env create gives confusing message if environment file does not exist

Try executing `conda env create -f does_not_exist.txt`.  The output I get is:

```
Using Anaconda API: https://api.anaconda.org

SpecNotFound: Can't process without a name
```

That is not super useful or meaningful.  I'd suggest it should say something obvious, like:

```
File not found: does_not_exist.txt
```