gaiacli tx send --to and --amount flags should be required

[alessio]

Summary of Bug

$ gaiacli tx send --chain-id=test --from=foo 
ERROR: decoding Bech32 address failed: must provide an address
$ gaiacli tx send --chain-id=test --from=foo --to cosmos19vmrnge8hh8le3jggajaggglgpq5v9asgxy3g7 
Password to sign with 'foo':
ERROR: {"codespace":1,"code":11,"abci_code":65547,"message":"Error{invalid coins}"}

The former should explain better what's wrong, e.g. mention the --to flag. The latter should just not happen.

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