Use DecimalAmount for errors and fix the equality for DecimalAmount

[TheQuantumPhysicist]

Currently, errors derived from thiserror (especially in the wallet) use Amount when errors occur. The problem with that is that the errors appear very unreadable for users, something in the form:

Wallet error: Wallet error: Coin selection error: Not enough funds got: Amount { val: 99999939400000 }, requested: Amount { val: 4000000014300000 }

We can use DecimalAmount instead, but the issue there is that it doesn't implement Eq or PartialEq because equality can be achieved even if the contents inside are not the same, where different amounts with different amounts of decimals can be equal.

We either fix this, or we create yet another type for amount errors that assumes equality is only for all equal.