No warning / error message when user is not registered

[s-tikhomirov]

Some commands require the user to be registered, but do not signal error when this is not the case.

For instance, calling show-balance before register:

✔ Balance of 0xc27cffb5c1be0f72b58b3639d5bad4a92e82ce57

Calling show-balance after register:

✔ Balance of 0xc27cffb5c1be0f72b58b3639d5bad4a92e82ce57
ℹ USD:	0.00
ℹ EUR:	0.00
ℹ GBP:	0.00
ℹ JPY:	0.00
ℹ CNY:	0.00
ℹ SGD:	0.00

In the first case, it may be useful to output a warning prompting the user to register.

Same for other functions that assume that the user is registered, e.g., create-fincontract. If the user is not registered, it starts executing and hangs.

Show an error message when appropriate: "This command requires you to register first: register".