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())?
 			};

src/cmd/wallet_args.rs

@@ -1000,6 +1000,11 @@ pub fn parse_contract_new_args(
 			"You can only specify receive or send, not both.",
 		)));
 	};
+	if receive.is_none() && send.is_none() {
+		return Err(ParseError::ArgumentError(String::from(
+			"You must specify either --receive or --send.",
+		)));
+	};
 	// TODO: verify this is correct e.g. which values are passed here by default etc.
 	let src_acct_name = Some(String::from(account));
 	let add_outputs = args.is_present("add-outputs");
@@ -1024,7 +1029,14 @@ pub fn parse_contract_new_args(
 		}
 	};
 	let make_outputs = match args.value_of("make-outputs") {
-		Some(v) => Some(String::from(v)),
+		Some(v) => {
+			// Validate each comma-separated amount here so a bad value returns a
+			// ParseError instead of panicking later in output_amounts().
+			for amt in v.split(',') {
+				parse_grin_amount(amt, "make-outputs")?;
+			}
+			Some(String::from(v))
+		}
 		None => None,
 	};
 
@@ -1096,7 +1108,14 @@ pub fn parse_contract_setup_args(
 		}
 	};
 	let make_outputs = match args.value_of("make-outputs") {
-		Some(v) => Some(String::from(v)),
+		Some(v) => {
+			// Validate each comma-separated amount here so a bad value returns a
+			// ParseError instead of panicking later in output_amounts().
+			for amt in v.split(',') {
+				parse_grin_amount(amt, "make-outputs")?;
+			}
+			Some(String::from(v))
+		}
 		None => None,
 	};
 	// TODO: should we catch if the person calls "--receive=5" when it should be "--send=5"?