api: do not crash with unknown blocknumber input

Proposed changes

Next methods were crashed with "pending" or future block number.

kaia.getStakingInfo(99999999)

kaia.getRewards(99999999), kaia.getRewards("pending")

> kaia.getRewards(99999999)
Error: method handler crashed
    at web3.js:6812:9(39)
    at send (web3.js:5223:62(29))
    at <eval>:1:16(3)

This PR fixes it, and now it returns currentblock information(pending) or unknown block error(future block).


> kaia.getRewards("pending")
{
burntFee: 0,
kef: 0,
kif: 0,
minted: 9600000000000000000,
proposer: 9600000000000000000,
rewards: {
0xf19ebf7b5766528b190e4a1b8073a0f1b6901c8b: 9600000000000000000
},
stakers: 0,
totalFee: 0
}

kaia.getRewards(99999999) Error: Unknown block at web3.js:6812:9(39) at send (web3.js:5223:62(29)) at :1:16(3)
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kaiachain / kaia