Use explicit result to string conversion

Accessing a list of constant strings would require updating the minimum required version. This change doesn't really justify that in my opinion.

resources/external_control.urscript

@@ -123,8 +123,16 @@ def targetWithinLimits(step_start, step_end, time):
 end
 
 def trajectory_result_to_str(trajectory_result):
-  trajectory_result_strings = ["SUCCESS", "CANCELED", "FAILURE"]
-  return trajectory_result_strings[trajectory_result]
+  if trajectory_result == TRAJECTORY_RESULT_SUCCESS:
+    return "SUCCESS"
+  end
+  if trajectory_result == TRAJECTORY_RESULT_CANCELED:
+    return "CANCELED"
+  end
+  if trajectory_result == TRAJECTORY_RESULT_FAILURE:
+    return "FAILURE"
+  end
+  return "UNKNOWN"
 end
 
 def terminateProgram():