pass result of createpsbt call back to calculateEstimatedFee

[djpnewton]

show the user why the fee estimation failed

[ben-kaufman]

Thanks @djpnewton
There's just a conflict to solve, although I'm not sure why is that change needed?

[djpnewton]

fixed the conflict.

the amount sanitation is needed because I was getting the exception string of an invalid "amount" back to the "calculate estimated fee" field. Although that is less likely to occur after a later commit where I use the form validation before calling the fee estimation endpoint