refactor: clean up namespace errors

The namespace error was poorly refactored and duplicated the prefix
string. The "rejected" case is now also tested.

router/src/server/http.rs

@@ -85,7 +85,7 @@ pub enum Error {
     /// name into a [`NamespaceId`].
     ///
     /// [`NamespaceId`]: data_types::NamespaceId
-    #[error("failed to resolve namespace ID: {0}")]
+    #[error(transparent)]
     NamespaceResolver(#[from] crate::namespace_resolver::Error),
 
     /// The router is currently servicing the maximum permitted number of
@@ -561,7 +561,7 @@ mod tests {
     use super::*;
     use crate::{
         dml_handlers::mock::{MockDmlHandler, MockDmlHandlerCall},
-        namespace_resolver::mock::MockNamespaceResolver,
+        namespace_resolver::{mock::MockNamespaceResolver, NamespaceCreationError},
     };
     use assert_matches::assert_matches;
     use data_types::{NamespaceId, NamespaceNameError};
@@ -1513,10 +1513,16 @@ mod tests {
                 crate::namespace_resolver::Error::Lookup(e)
             }),
             "failed to resolve namespace ID: \
-             failed to resolve namespace ID: \
              name [name] already exists",
         ),
 
+        (
+            NamespaceResolver(
+                crate::namespace_resolver::Error::Create(NamespaceCreationError::Reject("bananas".to_string()))
+            ),
+            "rejecting write due to non-existing namespace: bananas",
+        ),
+
         (
             RequestLimit,
             "this service is overloaded, please try again later",