The most important case here is if your glob isn't even a glob, it's
just a filename. But even if it was a glob, it's probably a mistake;
you'll probably end up with a confusing error due to an empty schema, or
a slightly less confusing error due to not having any operations.
Instead, let's just say outright that your glob didn't match any files.
Fixes#146.
Test plan:
This was a bit annoying to test via snapshot, so I just tested it
manually by modifying the example to use a glob that didn't match any
files, first for the schema then for the operations, and got errors like
```
bogus*.graphql did not match any files
exit status 1
example/main.go:68: running "go": exit status 1
```
