SendPayment: Improve handling of in-transit payments

[kaloudis]

Per Leo in Telegram:

One problem I've run into with Zeus is that a payment I'm making is being "held" for whatever reason by the recipient node, most likely by accident. In the current UI, a held payment is indistinguishable from a failed payment, so in my previous cases (and again today), i just try making another payment, which is again held, and another payment, which is again held. All three payments eventually go through and I'm stuck with three double-scoop ice cream cones.
Some error/notification that's different from success/failure might be useful here

We have some code to explicitly catch Zaplocker payments and display an appropriate message, but not in-transit messages in general (despite highlighting them on the main wallet view).

Should be able to make some changes here: https://github.com/ZeusLN/zeus/blob/master/stores/TransactionsStore.ts#L566