Optimize error messages

[zhangyelong]

Is your feature request related to a problem? Please describe.
Many error messages are confusing, almost every one asks what these mean and what they should do with them, e.g.

• ERROR: {"codespace":"sdk","code":19,"message":"no native fee token, expected native token is iris-atto"}
• ERROR: {"codespace":"sdk","code":17,"message":"equivalent gas price (1000000000000iris-atto) is less than threshold (6000000000000iris-atto)"}

Describe the solution you'd like

• ERROR: {"codespace":"sdk","code":19,"message":"fee is required to be specified with iris"}
• ERROR: {"codespace":"sdk","code":17,"message":"insufficient fee, fee = gasLimit * gasPrice(default 0.000006iris)"}

[shilei-wang]

confirm fixed