Enumerate mempool errors (#2615)

* Enumerate mempool errors.

* Update code formatting

* Allow dead code

Co-authored-by: Alfredo Garcia <oxarbitrage@gmail.com>

* Allow dead code

Co-authored-by: Alfredo Garcia <oxarbitrage@gmail.com>

* Add a new error

* Update error formatting

Co-authored-by: Alfredo Garcia <oxarbitrage@gmail.com>

* Remove TransactionQueryError

* Derive Copy and Clone

Co-authored-by: teor <teor@riseup.net>

* Remove the Copy trait

Co-authored-by: teor <teor@riseup.net>

* Rename enum variants

Co-authored-by: Alfredo Garcia <oxarbitrage@gmail.com>
Co-authored-by: teor <teor@riseup.net>
Co-authored-by: Deirdre Connolly <deirdre@zfnd.org>

zebrad/src/components/mempool.rs

@@ -1,5 +1,8 @@
 //! Zebra mempool.
 
+/// Mempool-related errors.
+pub mod error;
+
 mod crawler;
 
 pub use self::crawler::Crawler;

zebrad/src/components/mempool/error.rs

@@ -0,0 +1,25 @@
+//! Errors that can occur when manipulating transactions in the mempool.
+
+use thiserror::Error;
+
+#[derive(Error, Clone, Debug, PartialEq)]
+#[allow(dead_code)]
+pub enum MempoolError {
+    #[error("transaction already exists in mempool")]
+    InMempool,
+
+    #[error("transaction did not pass consensus validation")]
+    Invalid(#[from] zebra_consensus::error::TransactionError),
+
+    #[error("transaction is committed in block {0:?}")]
+    InBlock(zebra_chain::block::Hash),
+
+    #[error("transaction has expired")]
+    Expired,
+
+    #[error("transaction fee is too low for the current mempool state")]
+    LowFee,
+
+    #[error("transaction was not found in mempool")]
+    NotInMempool,
+}