Add description to asserts.

std/experimental/range/ndslice.d

@@ -927,9 +927,9 @@ auto byElement(size_t N, Range)(auto ref Slice!(N, Range) slice)
             Tuple!(size_t, size_t) opSlice(size_t pos : 0)(size_t i, size_t j)
             in   {
                 assert(i <= j,
-                    "opSlice!" ~ pos.stringof ~ ": left bound must be less then or equal right bound");
+                    "ByElement.opSlice: left bound must be less then or equal right bound");
                 assert(j - i <= _length,
-                    "opSlice!" ~ pos.stringof ~ ": difference between right and left bounds must be less then or equal length");
+                    "ByElement.opSlice: difference between right and left bounds must be less then or equal length");
             }
             body {
                 return typeof(return)(i, j);
@@ -1704,7 +1704,10 @@ public:
     Tuple!(size_t, size_t) opSlice(size_t dimension)(size_t i, size_t j)
         if (dimension < N)
     in   {
-        assert(i <= j && j - i <= _lengths[dimension]);
+        assert(i <= j,
+            "Slice.opSlice!" ~ dimension.stringof ~ ": left bound must be less then or equal right bound");
+        assert(j - i <= _lengths[dimension],
+            "Slice.opSlice!" ~ dimension.stringof ~ ": difference between right and left bounds must be less then or equal length");
     }
     body {
         return typeof(return)(i, j);