feat(generator): check if glob pattern is valid

[landreussi]

This patch adds a validation that checks if the glob pattern found any files.

That's a validation that does make sense for my use case, which consists of a proc macro that expands Rust types from Avro schemas, and a build script that does the same thing.

This modification could break existing use cases:
If no files were found, the clients of this crate will need to treat that case like this:

match g.generate(...) {
    Ok(_) => Ok(()),
    Err(Error::GlobPattern(..)) => Ok(()),
    Err(e) => Err(e),
}?;

This looks to me like this validation should exist in the crate, but let me know if you have any disagreements.

[leo-def]

Great job

[lerouxrgd]

Thanks for the PR !