cloud_storage: throw if can not create manifest cursor

It is not reasonable to continue work after this point. The absence of a
cursor cursor is interpreted as EOF in other parts of the system. Not
throwing makes it impossible to differentiate between "no more data
available" vs. "an error occurred".

This is required for an upcoming commit that fixes a timequery bug where
cloud storage returns "no offset found" instead of propagating an
internal error.

src/v/cloud_storage/remote_partition.cc

@@ -619,12 +619,10 @@ class partition_record_batch_reader_impl final
                 co_return;
             }
 
-            vlog(
-              _ctxlog.error,
+            throw std::runtime_error(fmt::format(
               "Failed to query spillover manifests: {}, query: {}",
               cur.error(),
-              query);
-            co_return;
+              query));
         }
         _view_cursor = std::move(cur.value());
         co_await _view_cursor->with_manifest(