Improve error reporting and logging.

* Use Binder ExceptionCode rather than StatusCode, so that we can
  include a message string.
* Use Display rather than Debug for formatting errors, to give more
  readable messages.

Test: atest VirtualizationTestCases
Change-Id: Id281f56609b798bcc52105dabe86cd040655b18d
diff --git a/virtualizationservice/src/crosvm.rs b/virtualizationservice/src/crosvm.rs
index 138236c..797011c 100644
--- a/virtualizationservice/src/crosvm.rs
+++ b/virtualizationservice/src/crosvm.rs
@@ -133,7 +133,7 @@
 
         // Delete temporary files.
         if let Err(e) = remove_dir_all(&self.temporary_directory) {
-            error!("Error removing temporary directory {:?}: {:?}", self.temporary_directory, e);
+            error!("Error removing temporary directory {:?}: {}", self.temporary_directory, e);
         }
     }