HttpBinaryCacheStore::getFile(): Don't throw an exception

This violates the noexcept specification.

Fixes #6445.

src/libstore/http-binary-cache-store.cc

@@ -161,7 +161,12 @@ protected:
     void getFile(const std::string & path,
         Callback<std::optional<std::string>> callback) noexcept override
     {
+        try {
             checkEnabled();
+        } catch (...) {
+            callback.rethrow();
+            return;
+        }
 
         auto request(makeRequest(path));