Add proper error messages in __check_input__() (#5042)

* Add proper error messages in `__check_input__()`

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Format update to pass linting error

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

torch_geometric/nn/conv/message_passing.py

@@ -180,9 +180,12 @@ def __check_input__(self, edge_index, size):
         the_size: List[Optional[int]] = [None, None]
 
         if isinstance(edge_index, Tensor):
-            assert edge_index.dtype == torch.long
-            assert edge_index.dim() == 2
-            assert edge_index.size(0) == 2
+            assert edge_index.dtype == torch.long, \
+                "edge_index.dtype is not of torch.long"
+            assert edge_index.dim() == 2, \
+                "edge_index.dim() is not equal to 2"
+            assert edge_index.size(0) == 2, \
+                "edge_index.size(0) is not equal to 2"
             if size is not None:
                 the_size[0] = size[0]
                 the_size[1] = size[1]