Trade: use consistent naming in an assert message.

src/Magnum/Trade/MeshData.h

@@ -524,7 +524,7 @@ class MAGNUM_TRADE_EXPORT MeshAttributeData {
                 /* We're *sure* the view is correct, so faking the view size */
                 /** @todo better ideas for the StridedArrayView API? */
                 {_data.pointer, ~std::size_t{}}, _vertexCount,
-                (CORRADE_CONSTEXPR_ASSERT(!_isOffsetOnly, "Trade::MeshAttributeData::data(): the attribute is a relative offset, supply a data array"), _stride)};
+                (CORRADE_CONSTEXPR_ASSERT(!_isOffsetOnly, "Trade::MeshAttributeData::data(): the attribute is offset-only, supply a data array"), _stride)};
         }
 
         /**

src/Magnum/Trade/Test/MeshDataTest.cpp

@@ -795,11 +795,13 @@ void MeshDataTest::constructAttributeWrongDataAccess() {
     CORRADE_VERIFY(!a.isOffsetOnly());
     CORRADE_VERIFY(b.isOffsetOnly());
 
+    a.data(positionData); /* This is fine, no asserts */
+
     std::ostringstream out;
     Error redirectError{&out};
     b.data();
     CORRADE_COMPARE(out.str(),
-        "Trade::MeshAttributeData::data(): the attribute is a relative offset, supply a data array\n");
+        "Trade::MeshAttributeData::data(): the attribute is offset-only, supply a data array\n");
 }
 
 constexpr Vector2 ArrayVertexData[3*4]