On main, there is no error if the schema file cannot be found

[johnmaguire]

Describe the bug
In v0.3.0 if you point to a schema file that does not exist you will be given an error:

unreadable schema file graph/schema.graphql: open graph/schema.graphql: no such file or directory

Past v0.3.0, you get a confusing error that the types are not known:

graph/genqlient.graphql:35: query-spec does not match schema: Unknown type "ActionInput".

To Reproduce

1. genqlient.yaml contains a schema path that does not exist: schema: graph/nonexistent.graphqls
2. Create an operations file that references types defined in the schema
3. Run go run github.com/Khan/genqlient

Expected behavior

unreadable schema file graph/schema.graphql: open graph/schema.graphql: no such file or directory

genqlient version
v0.3.1-0.20211022224026-a4aa6d9bb0f4

The problem cannot be reproduced on v0.3.0.

Additional context
I believe the behavior was introduced in this commit: 59b6df6

Since we are globbing, we end up with an empty list of files to iterate. Since we don't try to load any files, we never return an error that they can't be loaded.

I'm not sure the best fix here - perhaps try to detect if a line is a glob or not, and only call the glob function if it is? Or maybe a glob pattern that returns no results should result in an error - or warning.

[benjaminjkraft]

Ah, good find. We should just check explicitly that we got at least one schema, e.g. that there's at least one type defined, or at least one filename. As you point out, there's some question as to whether we should require each glob match at least one file as well, I'd lean towards yes, especially since it sounds from your description like we do that even if there is no real glob.