Remove CLI error strings from stellar-rpc-client

[leighmcculloch]

The stellar-rpc-client contains a CLI error string, that should really exist in the CLI itself. The stellar-rpc-client should be usable in other contexts without the CLI's error messages showing up.

For example:

soroban-rpc/cmd/crates/stellar-rpc-client/src/lib.rs

Lines 694 to 696 in 5c6d657

	Might need to fund account like:
	soroban config identity fund {address} --network <name>
	soroban config identity fund {address} --helper-url <url>"#