Improve Error Message on `ServiceUnavailable`

[AlexCatarino]

Expected Behavior

When the service is unavailable, Lean adds this information to the runtime error.

Actual Behavior

When the service is unavailable, Lean creates a brokerage message:
Error - Code: ServiceUnavailable - - Brokerage: TradierBrokerage
but the runtime error is raised by the following API call, which can return an empty message. This results in the following message:
Runtime Error:

syslog:

2022-09-22T23:41:32.4135621Z ERROR:: TradierBrokerage.Execute.accounts/6YA23301/orders(2): Parameters:  Response: <?xml version='1.0' encoding='UTF-8'?><fault><faultstring>Gateway Timeout</faultstring><detail><errorcode>messaging.adaptors.http.flow.GatewayTimeout</errorcode></detail></fault>
[  ...  Attempts to reconnect ... ]
2022-09-22T23:42:17.1942707Z ERROR:: Brokerage.OnMessage(): Error - Code: ServiceUnavailable - 
2022-09-22T23:42:17.1975736Z TRACE:: CloudLiveTradingResultHandler.BrokerageMessage(): Error - Code: ServiceUnavailable -  - Brokerage: TradierBrokerage
2022-09-22T23:42:17.2036159Z ERROR:: Extensions.SetRuntimeError(): Extensions.SetRuntimeError(): RuntimeError at 9/22/2022 11:42:16 PM UTC. Context: Brokerage Error System.Exception
2022-09-22T23:42:17.2764612Z TRACE:: Tradier.FetchOrders(): No orders found
2022-09-22T23:42:17.6945296Z ERROR:: TradierBrokerage.Execute.accounts/6YA23301/orders(2): Parameters:  Response: 
2022-09-22T23:42:17.6945789Z TRACE:: TradierBrokerage.Execute.accounts/6YA23301/orders(2): Attempting again...
2022-09-22T23:42:17.8017087Z ERROR:: AlgorithmManager.Run(): Stopping, encountered a runtime error at 9/22/2022 11:42:17 PM UTC.

System Information

Live trading in QuantConnect Cloud.

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