Do not fail if assigns PR author

[JamesWatling]

Hi just started using the action but we have another action that adds labels and then this action to assign the reviewer.

The problem arises if I am the member to be marked as a reviewer for that label I get the following error (assuming this is flowing through from github)

Review cannot be requested from pull request author

Do we think it's nice to handle this case and still add the other reviewers?