types: move mempool error for consistency (#6875)

This is a little change just to make things more consistent ahead of
the 0.35 release.

types/mempool.go

@@ -0,0 +1,55 @@
+package types
+
+import (
+	"errors"
+	"fmt"
+)
+
+var (
+	// ErrTxInCache is returned to the client if we saw tx earlier
+	ErrTxInCache = errors.New("tx already exists in cache")
+)
+
+// ErrTxTooLarge defines an error when a transaction is too big to be sent in a
+// message to other peers.
+type ErrTxTooLarge struct {
+	Max    int
+	Actual int
+}
+
+func (e ErrTxTooLarge) Error() string {
+	return fmt.Sprintf("Tx too large. Max size is %d, but got %d", e.Max, e.Actual)
+}
+
+// ErrMempoolIsFull defines an error where Tendermint and the application cannot
+// handle that much load.
+type ErrMempoolIsFull struct {
+	NumTxs      int
+	MaxTxs      int
+	TxsBytes    int64
+	MaxTxsBytes int64
+}
+
+func (e ErrMempoolIsFull) Error() string {
+	return fmt.Sprintf(
+		"mempool is full: number of txs %d (max: %d), total txs bytes %d (max: %d)",
+		e.NumTxs,
+		e.MaxTxs,
+		e.TxsBytes,
+		e.MaxTxsBytes,
+	)
+}
+
+// ErrPreCheck defines an error where a transaction fails a pre-check.
+type ErrPreCheck struct {
+	Reason error
+}
+
+func (e ErrPreCheck) Error() string {
+	return e.Reason.Error()
+}
+
+// IsPreCheckError returns true if err is due to pre check failure.
+func IsPreCheckError(err error) bool {
+	return errors.As(err, &ErrPreCheck{})
+}