Report error if session token is not present in complete_oauth_flow

[bendk]

We've been getting a bunch of No stored session token error reports and have been discussing this on slack. My theory is that somehow the oauth flow is completing without a session token. Maybe because the set_user_data is coming after the complete_oauth_flow call.

This tries to validate that theory by reporting errors when complete_oauth_flow is called without a session token. Note that this won't lead always lead to the error state if set_user_data is called shortly after. However it could, for example if some exception prevents the set_user_data call, or Firefox exits early.

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