Improve error when supplying established address to --gas-payer

[brentstone]

Currently, the output would look like this:

target/release/namadac transparent-transfer --source brent-multisig --target brent-0 --token nam --amount 100 --signing-keys brent-2,brent-3 --node $DO_NODE --gas-payer brent-multisig

The application panicked (crashed).
Message:  called `Result::unwrap()` on an `Err` value: "Couldn't look-up public key associated with \"brent-multisig\""
Location: crates/apps_lib/src/cli/context.rs:271

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
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