atomic: fix error message at invalid group

[marioidival]

Closes #1040

What does this PR does?

Add properly message error to atomic group transactions.

How to test?

Checklist

These are the criteria that every PR should meet, please check them off as you review them:

• [X] Include tests
• [X] Respect code style and lint
• [ ] Update swagger file (if needed)
• [ ] Update documentation (*.md) (if needed)

Starting test
    ✓ Init variables (4441ms)
    ✓ Check ETH L1 deposit (38350ms)
    ✓ Check ERC20 L1 deposit (44534ms)
    ✓ Check ETH L1 force exit (44260ms)
    ✓ Check ERC20 L1 force exit (48302ms)
    ✓ Check ETH withdrawal from firsts forceExits (16299ms)
    ✓ Check ERC20 withdrawal from firsts forceExits (12263ms)
    ✓ Check ETH N L1 force exits (38300ms)
    ✓ Check ETH withdrawal from previous N force exits (4107ms)
    ✓ Check single ETH L2 transfer (40267ms)
    ✓ Check single ERC20 L2 transfer (40212ms)
    ✓ Transfer ETH to a non-existent Bjj address (50324ms)
    ✓ Transfer ERC20 to a non-existent Bjj address (40313ms)
    ✓ Transfer ETH to hermez ethereum address (40327ms)
    ✓ Transfer ERC20 to hermez ethereum address (30195ms)
    ✓ Transfer ETH to non-existent ETH address (40334ms)
    ✓ Transfer ETH to hermez ethereum address from internal account (40239ms)
    ✓ Transfer ERC20 to hermez ethereum address from internal account (30208ms)
    ✓ Check multiple L2 ETH transfer in same batch (50779ms)
    ✓ Check multiple L2 ETH transfer in different batches (171580ms)
    ✓ Atomic txs (50478ms)

  21 passing (15m)