De-panic and validate contract CLI input handling

Bad input to the contract CLI could abort the process:
- a non-numeric --make-outputs value reached output_amounts(), which
  unwrap()s the float parse;
- `contract new` with neither --send nor --receive hit a panic in
  get_net_change;
- `contract sign` panicked ("No address to encrypt for") when given no
  --encrypt-for and an incoming slate with no embedded sender.

Validate --make-outputs and require exactly one of --send/--receive at
parse time (ParseError, before reaching the panicking paths), and return a
typed error instead of panicking when there is no address to encrypt for.
Verified at runtime: each now yields a clean "Invalid Arguments" error.

controller/src/command.rs

@@ -1887,8 +1887,10 @@ where
 			if args.counterparty_addr.is_some() {
 				args.counterparty_addr.unwrap()
 			} else {
-				if !slatepack.sender.is_some() {
-					panic!("No address to encrypt for. Contracts only support encrypted slates right now.");
+				if slatepack.sender.is_none() {
+					return Err(grin_wallet_libwallet::Error::GenericError(
+						"No address to encrypt for: pass --encrypt-for. Contracts only support encrypted slates.".into(),
+					));
 				}
 				String::try_from(&slatepack.sender.unwrap())?
 			};