Migrate to thiserror (#7177)

* Migrate to thiserror

* Discourage the use of other modules' Result alias

`io::Result` set a bad precedent. Don't import other `Result`
aliases.

ledger/src/block_error.rs

@@ -1,18 +1,25 @@
-#[derive(Debug, PartialEq)]
+use thiserror::Error;
+
+#[derive(Error, Debug, PartialEq)]
 pub enum BlockError {
     /// Block entries hashes must all be valid
+    #[error("invalid entry hash")]
     InvalidEntryHash,
 
     /// Blocks must end in a tick that has been marked as the last tick.
+    #[error("invalid last tick")]
     InvalidLastTick,
 
     /// Blocks can not have extra ticks or missing ticks
+    #[error("invalid tick count")]
     InvalidTickCount,
 
     /// All ticks must contain the same number of hashes within a block
+    #[error("invalid tick hash count")]
     InvalidTickHashCount,
 
     /// Blocks must end in a tick entry, trailing transaction entries are not allowed to guarantee
     /// that each block has the same number of hashes
+    #[error("trailing entry")]
     TrailingEntry,
 }