Fix EcsOperatorError, so it can be loaded from a picklefile

[ngwallace]

Had a similar issue as described here.
Found out that EcsOperatorError was not correctly implemented and couldn't be loaded from a pickle dump.
The proposed fix solves this issue. (for reference: https://stackoverflow.com/questions/41808912/cannot-unpickle-exception-subclass)

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
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[potiuk]

Nice. But static checks need fixing

[boring-cyborg]

Awesome work, congrats on your first merged pull request!