rfq: add structured oracle error codes

Formalizes the 'Code' of an 'OracleError' as an 'OracleErrorCode', and
adds a couple of values corresponding to structured error cases.

Author: jtobin

rfq/oracle.go

@@ -86,12 +86,26 @@ const (
 // service.
 type OracleError struct {
 	// Code is a code which uniquely identifies the error type.
-	Code uint8
+	Code OracleErrorCode
 
 	// Msg is a human-readable error message.
 	Msg string
 }
 
+// OracleErrorCode uniquely identifies the kinds of error an oracle may
+// return.
+type OracleErrorCode uint8
+
+const (
+	// ErrUnspecifiedOracleError represents the case where the oracle has
+	// declined to give a more specific reason for the error.
+	ErrUnspecifiedOracleError OracleErrorCode = iota
+
+	// ErrUnsupportedOracleAsset represents the case in which an oracle does
+	// not provide quotes for the requested asset.
+	ErrUnsupportedOracleAsset
+)
+
 // Error returns a human-readable string representation of the error.
 func (o *OracleError) Error() string {
 	// Sanitise price oracle error message by truncating to 255 characters.