box: fix unhelpful decoding of error messages into decimal numbers

Before this change the box backend could make errors like

    Error "not_found" (404): On-Behalf-Of User not found ([123 34 105 110
    118 97 108 105 100 95 117 115 101 114 95 105 100 34 58 123 34 105 100
    34 58 34 48 48 48 48 48 48 48 48 48 48 48 34 125 125])

This fixes it to produce this instead

    Error "not_found" (404): On-Behalf-Of User not found ({"invalid_user_id":{"id":"00000000000"}})

backend/box/api/types.go

@@ -52,7 +52,7 @@ func (e *Error) Error() string {
 		out += ": " + e.Message
 	}
 	if e.ContextInfo != nil {
-		out += fmt.Sprintf(" (%+v)", e.ContextInfo)
+		out += fmt.Sprintf(" (%s)", string(e.ContextInfo))
 	}
 	return out
 }