feat(generator/rust): re-export Result and Error

Application developers may need to name these types when using a
generated client. Re-exporting the types makes it possible to name
them unambiguously, something that may be hard (or impossible?) in
complex applications where there may be more than one version of `gax`.

generator/internal/rust/templates/crate/src/lib.rs.mustache

@@ -37,7 +37,8 @@ limitations under the License.
 pub mod model;
 
 {{#HasServices}}
-pub(crate) use gax::Result;
+pub use gax::Result;
+pub use gax::error::Error;
 
 {{! Google APIs often use angle brackets for <PLACEHOLDERS>, rustdoc does not like those. }}
 #[allow(rustdoc::invalid_html_tags)]

src/gax/src/lib.rs

@@ -30,7 +30,7 @@
 //! change both if needed.
 //! </div>
 
-/// An alias of [std::result::Result] where the error is always [crate::error::Error].
+/// An alias of [std::result::Result] where the error is always [Error][crate::error::Error].
 ///
 /// This is the result type used by all functions wrapping RPCs.
 pub type Result<T> = std::result::Result<T, crate::error::Error>;