Some Web3 RPC Handlers could panic

[tomtau]

System info: 1447bf6

Steps to reproduce:

1. start up Ethermint
2. send this payload to sendRawTransaction:

[]byte("\xf8\xe0m\"Z\x94\xa8L\xbd\xbb\xfe\x12\x81\xc5m\x10\x00m\x00m\x00\xffm\x80\x00\xb5\xb5\xb5\xb5\x00\x00\x00dmKKK\x1fKm\x00\xffmmmm{{{mmmmmmm]m\x80mmmmmmmmmmmmmb\x82\x82\x82~~~~]\xb8m\x82\x82mmm\xff\u007f\x10\xff\x89mm\xf0?\xf5mmmdmmm\x1bmmmmemmmm\x00\x04\x06mmmKKKmmmz\x8d\xe4)\xa9Dmmmmmm\xf3mmmmmmmmmmmmm-mmmmmmmmmm\xd0\xff\x00\x00\x01m\x80mm\xe8\xe8S\xe8\xe8mm\x00\x00\x04\x00mmmSe@m\x90\x90\x90\xc8\xc8Ȓ\x90\x90\x8e\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90mmm\xb5hm\x00mmkmm{")

Expected behavior:
graceful rejection of the invalid input

Actual behavior:

{"jsonrpc":"2.0","id":1,"error":{"code":-32000,"message":"method handler crashed"}}

Additional info:

Ethermint calls sdk.NewIntFromBigInt on untrusted user inputs (e.g. tx.Value()) and this method panics if the integer has more than 256 bits:
https://github.com/cosmos/cosmos-sdk/blob/master/types/int.go#L112

[yihuang]

I'll work on this one.