FTX: Add REST subaccount support (#653)

* FTX: Add REST API subaccount support

* Add API key check to GetSubaccounts

* Fix missing comment and expand to rest of the codebase

* Address glorious nits

* Address various nits

* Fix ZB typo

https://www.zb.com/api#hsptccieyyqomlp

backtester/common/common_types.go

@@ -39,7 +39,8 @@ var (
 	// ErrNilArguments is a common error response to highlight that nils were passed in
 	// when they should not have been
 	ErrNilArguments = errors.New("received nil argument(s)")
-	ErrNilEvent     = errors.New("nil event received")
+	// ErrNilEvent is a common error for whenever a nil event occurs when it shouldn't have
+	ErrNilEvent = errors.New("nil event received")
 )
 
 // EventHandler interface implements required GetTime() & Pair() return

backtester/eventhandlers/strategies/base/base_types.go

@@ -2,6 +2,7 @@ package base
 
 import "errors"
 
+// Error vars related to strategies and invalid config settings
 var (
 	ErrCustomSettingsUnsupported          = errors.New("custom settings not supported")
 	ErrSimultaneousProcessingNotSupported = errors.New("does not support simultaneous processing and could not be loaded")