Reject a glob that matches no files

[benjaminjkraft]

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

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable (see above)
• Included a link to the issue fixed, if applicable
• Included documentation, for new features (n/a)
• Added an entry to the changelog (n/a, fixing a bug not in a release)

[StevenACoffman]

Hmmm, yeah tricky to test, so fine to just do it without coverage.

[csilvers]

Seems good to me!