Tidy error when access denied to chipflow endpoint

[robtaylor]

currently invalid credentials gives a confusing error:

INFO:chipflow_lib.steps.silicon:Submitting 85a9c16 for project chipflow-examples
ERROR:chipflow_lib.steps.silicon:Request failed with status code 401
ERROR:chipflow_lib.steps.silicon:Request URL: https://build-staging.chipflow.org/build/submit
ERROR:chipflow_lib.steps.silicon:Request headers: {'User-Agent': 'python-requests/2.32.3', 'Accept-Encoding': 'gzip, deflate', 'Accept': '*/*', 'Connection': 'keep-alive', 'Content-Length': '5467307', 'Content-Type': 'multipart/form-data; boundary=69534e594d0b891d4b57ee86f22f29b1', 'Authorization': 'REDACTED'}
ERROR:chipflow_lib.steps.silicon:Request data: {'projectId': 'chipflow-examples', 'name': '85a9c16'}
ERROR:chipflow_lib.steps.silicon:Response headers: {'content-type': 'application/json', 'vary': 'Cookie', 'x-cloud-trace-context': '32f4428e9ebedc0d4b4fcdf3807644e8', 'date': 'Tue, 29 Apr 2025 19:53:[17](https://github.com/ChipFlow/chipflow-examples/actions/runs/14740082325/job/41375667293#step:8:18) GMT', 'server': 'Google Frontend', 'Content-Length': '74'}
ERROR:chipflow_lib.steps.silicon:Response body: {'message': 'Unauthorized access. Please provide valid credentials.'}
Error while executing `silicon submit`: Failed to submit design: {'message': 'Unauthorized access. Please provide valid credentials.'}

This dhould probably just be:

Error while executing `silicon submit`: Failed to submit design: 
'Unauthorized access. Please provide valid credentials.'