Improve Error Message for Invalid Credentials

Similar to https://github.com/QuantConnect/Lean.Brokerages.Coinbase/issues/16

Expected Behavior

401 Unauthorized explain that credentials are invalid.

Actual Behavior

We receive the following message which doesn't explain 401:

During the algorithm initialization, the following exception has occurred: Error getting cash balance from brokerage: CoinbaseApiClient.ExecuteRequest failed: [401] Unauthorized, Content: Unauthorized

Potential Solution

If the error code is 401. Ask user to verify credentials.
API Private Key is tricky since members need to remove \n which might lead to human error.

Reproducing the Problem

Deploy with old/invalid credentials.

Checklist

• I have completely filled out this template
• I have confirmed that this issue exists on the current master branch
• I have confirmed that this is not a duplicate issue by searching issues
• I have provided detailed steps to reproduce the issue

Start at CoinbaseApiClient.ExecuteRequest and reproduce the failure with old or invalid credentials. Trace how the 401 response becomes the initialization exception, then make the resulting message explain that credentials may be invalid and mention the private-key newline concern. Done means the invalid-credential scenario gives actionable guidance instead of only “Unauthorized”.