routing: map insufficient local bandwidth error to no path

With mpp it isn't possible anymore for findPath to determine that there
isn't enough local bandwidth. The full payment amount isn't known at
that point.

In a follow-up, this payment outcome can be reintroduced on a higher
level (payment lifecycle).

routing/pathfind.go

@@ -478,7 +478,7 @@ func findPathInternal(
 			return nil, err
 		}
 		if max < amt {
-			return nil, errInsufficientBalance
+			return nil, errNoPathFound
 		}
 	}