refactor: plaid connection

[batonac]

This makes the plaid bank connector much more performant and less error-prone.

• plaid_access_token is updated in the database directly via frappe.db.set_value instead of attempting to loading and save the Bank doc.
• The try/except clause in add_institution is expanded to include all updates to the plaid_access_token, to catch any errors related to the given bank_name
• add_institution is refactored to return just the bank_name instead of the bank doc, since only the bank_name is used in the subsequent add_bank_accounts function.
• Throw a dedicated error if bank_name is not returned in the Plaid response.

[stale]

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[batonac]

Can I get a review please?

[batonac]

@ankush, @deepeshgarg007, @casesolved-co-uk, any comments to help this move forward?

[casesolved-co-uk]

1. Plaid have withdrawn their free access via their development environment, so their lowest production plan is around $1500 per month making this integration pretty pointless unless you're setting up as a data warehouse
2. There's nothing broken; working, tested, mature code is always better than new untested unnecessary changes

[batonac]

Thanks for the review @casesolved-co-uk.

Re:

1. Plaid have withdrawn their free access via their development environment, so their lowest production plan is around $1500 per month making this integration pretty pointless unless you're setting up as a data warehouse

I think you might be misunderstanding Plaid's new fee structure. At least here in the US, they're charging $0.3/account/mo for production access to Transaction Sync.

2. There's nothing broken; working, tested, mature code is always better than new untested unnecessary changes

Plaid doesn't always return a bank account from Plaid.create--sometimes there's an error. The existing code has no error handling for this scenario. What's broken in that case is the user experience.

[casesolved-co-uk]

I guess their UK offering is different to the US then.
I suggest you post a bug with an error log that your PR fixes.