Resolve StreamError ArithmeticOverflow merge conflict

Author: akinerin

contracts/stream_contract/src/errors.rs

@@ -37,4 +37,9 @@ pub enum StreamError {
     StreamAlreadyPaused = 14,
     /// Operation requires an active stream, but the stream is inactive (cancelled or completed).
     StreamNotActive = 15,
+    /// An amount or timestamp calculation exceeded the range of its type.
+    ///
+    /// Returned instead of letting `overflow-checks` panic and abort the whole
+    /// transaction, so callers get a typed failure they can handle.
+    ArithmeticOverflow = 16,
 }