Add support for BPF program custom errors (#5743)

* Add support for BPF program custom errors

* Rename SOL_SUCCESS -> SUCCESS

programs/bpf/rust/external_spend/src/lib.rs

@@ -6,11 +6,10 @@ use solana_sdk::entrypoint::*;
 use solana_sdk::pubkey::Pubkey;
 
 entrypoint!(process_instruction);
-fn process_instruction(_program_id: &Pubkey, ka: &mut [SolKeyedAccount], _data: &[u8]) -> bool {
+fn process_instruction(_program_id: &Pubkey, ka: &mut [SolKeyedAccount], _data: &[u8]) -> u32 {
     // account 0 is the mint and not owned by this program, any debit of its lamports
     // should result in a failed program execution.  Test to ensure that this debit
     // is seen by the runtime and fails as expected
     *ka[0].lamports -= 1;
-
-    true
+    SUCCESS
 }