Make list_chunkmanagers more resilient to broken entrypoints

As I'm a developing my custom chunk manager, I'm often checking out
between my development branch and production branch breaking the
entrypoint.

This made xarray impossible to import unless I re-ran `pip install -e . -vv`
which is somewhat tiring.

This should help xarray be more resilient in other software's bugs in
case they install malformed entrypoints

Example:

```python
>>> from xarray.core.parallelcompat import list_chunkmanagers

>>> list_chunkmanagers()
<ipython-input-3-19326f4950bc>:1: UserWarning: Failed to load entrypoint MyChunkManager due to No module named 'my.array._chunkmanager'. Skipping.
  list_chunkmanagers()
{'dask': <xarray.core.daskmanager.DaskManager at 0x7f5b826231c0>}
```

Author: hmaarrfk

xarray/core/parallelcompat.py

@@ -22,6 +22,7 @@
 import numpy as np
 
 from xarray.core.pycompat import is_chunked_array
+from xarray.core.utils import emit_user_level_warning
 
 T_ChunkedArray = TypeVar("T_ChunkedArray", bound=Any)
 
@@ -57,9 +58,15 @@ def load_chunkmanagers(
 ) -> dict[str, ChunkManagerEntrypoint]:
     """Load entrypoints and instantiate chunkmanagers only once."""
 
-    loaded_entrypoints = {
-        entrypoint.name: entrypoint.load() for entrypoint in entrypoints
-    }
+    loaded_entrypoints = {}
+    for entrypoint in entrypoints:
+        try:
+            loaded_entrypoints[entrypoint.name] = entrypoint.load()
+        except ModuleNotFoundError as e:
+            emit_user_level_warning(
+                f"Failed to load entrypoint {entrypoint.name} due to {e}. Skipping.",
+            )
+            pass
 
     available_chunkmanagers = {
         name: chunkmanager()