return errors when invalid transaction submitted

[johannbarbie]

when using json rpc make sure to return error, if there is any.

[eshavkun]

tendermint (called by sendTx.js helper) returns following in case of error in spending condition:

   { check_tx: { code: 2, log: '[object Object]' },
     deliver_tx: {},
     hash:
      'ED364A322874B9415B757BD09382838182DCF8AA1681F14C5E60602CFD9FB8AF',
     height: '0' } }```
The problem here is that log part is not an actual object but a string (saying `'[object Object]'`), needed to fix logging in https://github.com/leapdao/leap-node/blob/master/lotion/lib/abci-app.js#L46

[troggy]

not a bounty — not defined properly

[johannbarbie]

not a bounty — not defined properly

[troggy]

why closing these?

[johannbarbie]

ok, then i'll only remove the bounty labels.