Notify users about failed V5 update.

[budzanowski]

When the Pinterest plugin will be update to the V5 version a token exchange call will be made.
If the token exchange call will fail this will leave the Pinterest integration in a unusable state.

The next course of action for merchants would be to redo the authentication manually. In order for them to do the authentication they have to know about the issue.

Tasks

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1. Notify merchant via notice that the update procedure failed #923
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2. Notify merchant via UI that the update procedure failed #924
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[budzanowski]

All Task done. Closing.