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.

programs/vest/Cargo.toml

@@ -18,6 +18,7 @@ serde = "1.0.103"
 serde_derive = "1.0.103"
 solana-sdk = { path = "../../sdk", version = "0.22.0" }
 solana-config-program = { path = "../config", version = "0.22.0" }
+thiserror = "1.0"
 
 [dev-dependencies]
 solana-runtime = { path = "../../runtime", version = "0.22.0" }

programs/vest/src/vest_instruction.rs

@@ -9,10 +9,14 @@ use solana_sdk::{
     pubkey::Pubkey,
     system_instruction,
 };
+use thiserror::Error;
 
-#[derive(Debug, Clone, PartialEq, FromPrimitive)]
+#[derive(Error, Debug, Clone, PartialEq, FromPrimitive)]
 pub enum VestError {
+    #[error("destination missing")]
     DestinationMissing,
+
+    #[error("unauthorized")]
     Unauthorized,
 }