improve ethatomiswap tool

+ clearer error messages (by pre-validating method requirements),
  it makes the commands slightly slower, but gives clearer errors
+ improved authentication flow:
  + (a) authenticate by signing from the CLI itself:
    give path to fileKey as a flag and enter securily the
    passphrase using the STDIN to decrypt that file
  + (b) or give a (by the daemon known) account address,
    as to use it to sign with that account from the daemon
  + (c) or give no account info at all, and use the first found account
    address instead (works best if only one account is known by the daemon)

Note: (b) and (c) is not as secure, as it relies that your daemon
      has the accounts unlocked, making it possible for anyone
      that can access its RPC endpoints to use your unlocked accounts