confusing message about instance name with simple typo

[Ian Johnson]

while trying to install the "scummvm" snap, I tried this:

```
$ snap install scummVM
error: cannot install "scummVM": invalid instance name: invalid snap name: "scummVM"
```

which was very confusing because there wasn't a "_" so I didn't think an instance name would have been specified, but it turns out that any capitalized letters get interpreted as the "instance name":

```
$ snap install hello-worldCAPS
error: cannot install "hello-worldCAPS": invalid instance name: invalid snap name:
       "hello-worldCAPS"
```

We should fix the message to just complain about capital letters instead of an instance name.