Retry with second lambda if connection error

Note, we assume whenever there is a `FunctionError` that there will be a
payload that contains an `errorMessage` key. It's implied implicitely in
the docs but it's not very explicit.

https://docs.aws.amazon.com/lambda/latest/dg/API_Invoke.html#API_Invoke_ResponseSyntax

app/clients/cbc_proxy.py

@@ -113,7 +113,10 @@ class CBCProxyClientBase(ABC):
         result = self._invoke_lambda(self.lambda_name, payload_bytes)
 
         if 'FunctionError' in result:
-            raise CBCProxyException('Function exited with unhandled exception')
+            if result['Payload']['errorType'] == "CBCNewConnectionError":
+                result = self._invoke_lambda(self.failover_lambda_name, payload_bytes)
+            else:
+                raise CBCProxyException('Function exited with unhandled exception')
 
         return result