Remove panics in dot pallet (paritytech#330)

* Don't panic

* Add more comments

parachain/pallets/dot-app/src/lib.rs

@@ -87,7 +87,12 @@ decl_event!(
 
 decl_error! {
 	pub enum Error for Module<T: Config> {
-
+		/// Illegal conversion between native and wrapped DOT.
+		///
+		/// In practice, this error should never occur under the conditions
+		/// we've tested. If however the bridge or the peer Ethereum contract
+		/// is exploited, then all bets are off.
+		Overflow
 	}
 }
 
@@ -98,6 +103,8 @@ decl_module! {
 
 		fn deposit_event() = default;
 
+		// Verify that `T::Decimals` is 10 (DOT), or 12 (KSM) to guarantee
+		// safe conversions between native and wrapped DOT.
 		fn integrity_test() {
 			sp_io::TestExternalities::new_empty().execute_with(|| {
 				let allowed_decimals: &[u32] = &[10, 12];
@@ -115,10 +122,7 @@ decl_module! {
 
 			T::Currency::transfer(&who, &Self::account_id(), amount, AllowDeath)?;
 
-			let amount_wrapped = match wrap::<T>(amount, T::Decimals::get()) {
-				Some(value) => value,
-				None => panic!("Runtime is misconfigured"),
-			};
+			let amount_wrapped = wrap::<T>(amount, T::Decimals::get()).ok_or(Error::<T>::Overflow)?;
 
 			let message = OutboundPayload {
 				sender: who.clone(),
@@ -139,10 +143,7 @@ decl_module! {
 				return Err(DispatchError::BadOrigin.into());
 			}
 
-			let amount_unwrapped = match unwrap::<T>(amount, T::Decimals::get()) {
-				Some(value) => value,
-				None => panic!("Runtime is misconfigured"),
-			};
+			let amount_unwrapped = unwrap::<T>(amount, T::Decimals::get()).ok_or(Error::<T>::Overflow)?;
 
 			let recipient = T::Lookup::lookup(recipient)?;
 			T::Currency::transfer(&Self::account_id(), &recipient, amount_unwrapped, KeepAlive)?;