Return proper error to CLI for all expected scenarios

When a CLI requests a spot price have some errors that are expected, where we can provide a proper error message for the CLI:
- Balance of ASB too low
- Buy amount sent by CLI exceeds maximum buy amount accepted by ASB
- ASB is running in maintenance mode and does not accept incoming swap requests

All of these errors returns a proper error to the CLI and prints a warning in the ASB logs.
Any other unexpected error will result in closing the channel with the CLI and printing an error in the ASB logs.

swap/src/monero.rs

@@ -192,12 +192,6 @@ pub struct InsufficientFunds {
     pub actual: Amount,
 }
 
-#[derive(Debug, Clone, Copy, thiserror::Error)]
-#[error("The balance is too low, current balance: {balance}")]
-pub struct BalanceTooLow {
-    pub balance: Amount,
-}
-
 #[derive(thiserror::Error, Debug, Clone, PartialEq)]
 #[error("Overflow, cannot convert {0} to u64")]
 pub struct OverflowError(pub String);