Keystore 2.0 error module

This patch provides some convenience methods for error handling in
Keystore 2.0. Specifically, a way to convert errors into messages to the
client and logging errors.

Test: keystore2_test
Bug: 159378374
Change-Id: Ifa6b5745533863bfd76319bc991748a47453d31e
diff --git a/keystore2/src/lib.rs b/keystore2/src/lib.rs
index 97c1839..d2c55a7 100644
--- a/keystore2/src/lib.rs
+++ b/keystore2/src/lib.rs
@@ -13,3 +13,5 @@
 // limitations under the License.
 
 //! This crate implements the Android Keystore 2.0 service.
+
+pub mod error;