Preserve existing Error values when converting from anyhow::Error. (#118)

Falls back to internal error conversion otherwise.

Signed-off-by: Ben Brandt <benjamin.j.brandt@gmail.com>

Author: benbrandt

rust/error.rs

@@ -206,7 +206,10 @@ impl Display for Error {
 
 impl From<anyhow::Error> for Error {
     fn from(error: anyhow::Error) -> Self {
-        Error::into_internal_error(&*error)
+        match error.downcast::<Self>() {
+            Ok(error) => error,
+            Err(error) => Error::into_internal_error(&*error),
+        }
     }
 }