`decimal.InvalidOperation` is a special case of a float parsing error, which
presumably should be handled in the same way as a general parsing error,
rather than blow up.

Alternatives include: logging the error, or re-raising with more information.

Example log output:
```
    File "/home/travis/build/bitcoin/bitcoin/ci/scratch/build/bitcoin-i686-pc-linux-gnu/test/functional/test_framework/test_framework.py", line 603, in sync_all
      self.sync_blocks(nodes)
    File "/home/travis/build/bitcoin/bitcoin/ci/scratch/build/bitcoin-i686-pc-linux-gnu/test/functional/test_framework/test_framework.py", line 568, in sync_blocks
      best_hash = [x.getbestblockhash() for x in rpc_connections]
    File "/home/travis/build/bitcoin/bitcoin/ci/scratch/build/bitcoin-i686-pc-linux-gnu/test/functional/test_framework/test_framework.py", line 568, in <listcomp>
      best_hash = [x.getbestblockhash() for x in rpc_connections]
    File "/home/travis/build/bitcoin/bitcoin/ci/scratch/build/bitcoin-i686-pc-linux-gnu/test/functional/test_framework/test_node.py", line 571, in __call__
      return self.cli.send_cli(self.command, *args, **kwargs)
    File "/home/travis/build/bitcoin/bitcoin/ci/scratch/build/bitcoin-i686-pc-linux-gnu/test/functional/test_framework/test_node.py", line 639, in send_cli
      return json.loads(cli_stdout, parse_float=decimal.Decimal)
    File "/usr/lib64/python3.6/json/__init__.py", line 367, in loads
      return cls(**kw).decode(s)
    File "/usr/lib64/python3.6/json/decoder.py", line 339, in decode
      obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    File "/usr/lib64/python3.6/json/decoder.py", line 355, in raw_decode
      obj, end = self.scan_once(s, idx)
  decimal.InvalidOperation: [<class 'decimal.InvalidOperation'>]
```
See: https://travis-ci.org/github/bitcoin/bitcoin/jobs/713502326

laanwj:
  ACK 82fc4017b774aaff8799c2b6e8ba5370d94dbf4d