Improve "insufficient funds" error. Add link to Buy ETH

#Description
As a user, I want to have more specific “insufficient funds” messages and have link to Buy ETH, so I can finish my transaction

#Situation
GIVEN: I am confirming a transaction (transaction confirmation, approve, etc)
WHEN: I have insufficient funds
THEN: I should get an alert notifying if I have insufficient token balance or insufficient ETH to pay gas
AND: Have a link to Buy ETH (be directed to onramp flow) in order to to finish the transaction

Trace the transaction confirmation and approval flows that produce insufficient-funds alerts, then identify how token balance and ETH gas balance are currently checked. Update the relevant alerts to distinguish those cases and include a Buy ETH link that enters the onramp flow; verify confirmation, approval, token-shortfall, and gas-shortfall scenarios.

Written by the indexing model from the issue text.