Add no funds error in faucet

[ecioppettini]

Return a 503 http status (Service unavailable) and a message when the faucet account has no funds to make the required transaction.

Note that as the faucet can only support one transaction at a time (because it queries the node for spending counter and balance), it will return a success message until the transaction that spends the funds is accepted by the node.

closes #87

[jpcapurro-atixlabs]

Looks good to me.

tests performed (LOVELACES_TO_GIVE==300 ):

send-money: happy path

• [ x ] faucet funds > 310
• [ x ] no pending txs
• [ x ] curl -X POST localhost:3000/send-money/receiver-address
• [ x ] receiver-address balanceincreases by 300

send-money: not enough funds for money given

• [ x ] faucet funds == 0
• [ x ] no pending txs
• [ x ] curl -X POST localhost:3000/send-money/receiver-address
• [ x ] faucet returns 503

send-money: not enough funds for fee + money given (but enough funds for money
given)

• [ x ] faucet funds == 300
• [ x ] no pending txs
• [ x ] curl -X POST localhost:3000/send-money/receiver-address
• [ x ] faucet returns 503