Hide error enum variants from cxx-gen public api

We can expose more detail on the error as the need arises, but start
with an opaque error type for now.
diff --git a/gen/src/mod.rs b/gen/src/mod.rs
index 320ff66..a5a3780 100644
--- a/gen/src/mod.rs
+++ b/gen/src/mod.rs
@@ -10,7 +10,7 @@
 #[cfg(test)]
 mod tests;
 
-pub use self::error::Error;
+pub(super) use self::error::Error;
 use self::error::{format_err, Result};
 use self::file::File;
 use crate::syntax::report::Errors;