Restore the exit codes to the v0.42.0 values (#2897)

The only thing different is the meaning of the 103 exit code. It used to be `GenericEngine`, i.e. "the test died from an unknown cause", and will now be used for the well-defined `ScriptStoppedFromRESTAPI` error.

errext/exitcodes/codes.go

@@ -14,10 +14,10 @@ const (
 	SetupTimeout             ExitCode = 100
 	TeardownTimeout          ExitCode = 101
 	GenericTimeout           ExitCode = 102 // TODO: remove?
-	CannotStartRESTAPI       ExitCode = 103
+	ScriptStoppedFromRESTAPI ExitCode = 103
 	InvalidConfig            ExitCode = 104
 	ExternalAbort            ExitCode = 105
-	ScriptStoppedFromRESTAPI ExitCode = 106
+	CannotStartRESTAPI       ExitCode = 106
 	ScriptException          ExitCode = 107
 	ScriptAborted            ExitCode = 108
 )