Clarify source of error message in stream testing. (rapidsai#13541)

This PR changes the error message from stream testing to identify that it's being raised by a cudf test utility. I didn't recognize the error text at first, and assumed it was an error from CUDA or nvbench.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - David Wendt (https://github.com/davidwendt)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: rapidsai#13541

cpp/tests/utilities/identify_stream_usage.cpp

@@ -161,9 +161,9 @@ void check_stream_and_error(cudaStream_t stream)
 #endif  // __GNUC__
     char const* env_stream_error_mode{std::getenv("GTEST_CUDF_STREAM_ERROR_MODE")};
     if (env_stream_error_mode && !strcmp(env_stream_error_mode, "print")) {
-      std::cout << "Found unexpected stream!" << std::endl;
+      std::cout << "cudf_identify_stream_usage found unexpected stream!" << std::endl;
     } else {
-      throw std::runtime_error("Found unexpected stream!");
+      throw std::runtime_error("cudf_identify_stream_usage found unexpected stream!");
     }
   }
 }