Merge InstructionError and ProgramError

From the user's perspective, it's just an instruction error.
For program-specific errors, we still have
InstructionError::CustomError.

core/src/blocktree_processor.rs

@@ -459,7 +459,7 @@ mod tests {
             entries.push(entry);
 
             // Add a second Transaction that will produce a
-            // ProgramError<0, ResultWithNegativeLamports> error when processed
+            // InstructionError<0, ResultWithNegativeLamports> error when processed
             let keypair2 = Keypair::new();
             let tx = SystemTransaction::new_account(&keypair, &keypair2.pubkey(), 42, blockhash, 0);
             let entry = Entry::new(&last_entry_hash, 1, vec![tx]);