vm, ethapi: add limit option to traceTransaction

that specifies the maximum number of elements in the `structLogs`
output.  This option is useful for debugging a transaction that
involves a large number of repetition.

For example,
```
debug.traceTransaction(tx, {disableStorage: true, limit: 2})
```
shows at most the first two steps in the `structLogs`.

core/vm/vm.go

@@ -183,7 +183,10 @@ func (evm *EVM) Run(contract *Contract, input []byte) (ret []byte, err error) {
 		mem.Resize(newMemSize.Uint64())
 		// Add a log message
 		if evm.cfg.Debug {
-			evm.cfg.Tracer.CaptureState(evm.env, pc, op, contract.Gas, cost, mem, stack, contract, evm.env.Depth(), nil)
+			err = evm.cfg.Tracer.CaptureState(evm.env, pc, op, contract.Gas, cost, mem, stack, contract, evm.env.Depth(), nil)
+			if err != nil {
+				return nil, err
+			}
 		}
 
 		if opPtr := evm.jumpTable[op]; opPtr.valid {