[REQ] Add status codes and error message to responses

[wslyvh]

Is your feature request related to a problem? Please describe.
The Raiden API describes all expected responses and status codes for each call. The client only returns a generic error 'invalid response' right now.

Describe the solution you'd like
Include more descriptive error messages, incl. its status code to make troubleshooting more efficient.

E.g. for registering a token, the following responses can be expected:

• 201 Created – A token network for the token has been successfully created.
• 402 Payment Required – Insufficient ETH to pay for the gas of the register on-chain transaction
• 404 Not Found – The given token address is invalid.
• 409 Conflict – The token was already registered before, or The registering transaction failed.
• 501 Not Implemented – Registering a token only works on testnet temporarily. On mainnet this error is returned.

See the Raiden API documentation: https://raiden-network.readthedocs.io/en/latest/rest_api.html