Remove InsnFunctionStepResult::Busy

we don't need all these busy variants, let's just handle
LimboError::Busy

core/incremental/expr_compiler.rs

@@ -435,11 +435,6 @@ impl CompiledExpression {
                                 "Expression evaluation was interrupted".to_string(),
                             ));
                         }
-                        crate::vdbe::execute::InsnFunctionStepResult::Busy => {
-                            return Err(crate::LimboError::InternalError(
-                                "Expression evaluation encountered busy state".to_string(),
-                            ));
-                        }
                         crate::vdbe::execute::InsnFunctionStepResult::Step => {
                             pc = state.pc as usize;
                         }

core/vdbe/execute.rs

@@ -159,7 +159,6 @@ pub enum InsnFunctionStepResult {
     IO(IOCompletions),
     Row,
     Interrupt,
-    Busy,
     Step,
 }
 
@@ -2205,11 +2204,7 @@ pub fn op_transaction(
                 !conn.is_nested_stmt.get(),
                 "nested stmt should not begin a new read transaction"
             );
-            let res = pager.begin_read_tx();
-            if let Err(LimboError::Busy) = res {
-                return Ok(InsnFunctionStepResult::Busy);
-            }
-            res?;
+            let res = pager.begin_read_tx()?;
         }
 
         if updated && matches!(new_transaction_state, TransactionState::Write { .. }) {
@@ -2227,7 +2222,7 @@ pub fn op_transaction(
                     conn.transaction_state.replace(TransactionState::None);
                 }
                 assert_eq!(conn.transaction_state.get(), current_state);
-                return Ok(InsnFunctionStepResult::Busy);
+                return Err(LimboError::Busy);
             }
             if let IOResult::IO(io) = begin_w_tx_res? {
                 // set the transaction state to pending so we don't have to

core/vdbe/mod.rs

@@ -675,7 +675,7 @@ impl Program {
                     // Instruction interrupted - may resume at same PC
                     return Ok(StepResult::Interrupt);
                 }
-                Ok(InsnFunctionStepResult::Busy) => {
+                Err(LimboError::Busy) => {
                     // Instruction blocked - will retry at same PC
                     return Ok(StepResult::Busy);
                 }