Use context()? instead of directly unwrap()-ing.

Test: m
Change-Id: Ib353f1eb7a3ea0002b40c4c8601d7c633b7fb571
diff --git a/virtualizationservice/src/aidl.rs b/virtualizationservice/src/aidl.rs
index edca883..050b89c 100644
--- a/virtualizationservice/src/aidl.rs
+++ b/virtualizationservice/src/aidl.rs
@@ -128,8 +128,8 @@
         .context("failed to create idsig")?;
 
     let mut output = clone_file(idsig_fd)?;
-    output.set_len(0).unwrap();
-    sig.write_into(&mut output).unwrap();
+    output.set_len(0).context("failed to set_len on the idsig output")?;
+    sig.write_into(&mut output).context("failed to write idsig")?;
     Ok(())
 }