[transaction confirmation] Show alert on confirmation when there is already a pending transaction

[rachelcope]

#Description
As a user, I want to be alerted if I an submitting a new transaction but already have a transaction pending and be prompted to to speed up existing transaction, so my new transaction is not stalled

#Situation
GIVEN: I am confirming a transaction (transaction confirmation, approve, etc)
WHEN: When I already have a transaction pending
THEN: I should get an alert notifying me of a pending transaction and prompting me to speed up existing transaction

Error use cases