improve the error message when pooch is not installed (pydata#5312)

* improve the error message when pooch is not installed

* add pooch to "pip install xarray[io]" [skip-ci]

* don't mention of pypi and conda [skip-ci]

setup.cfg

@@ -90,6 +90,7 @@ io =
     cftime
     rasterio
     cfgrib
+    pooch
     ## Scitools packages & dependencies (e.g: cartopy, cf-units) can be hard to install
     # scitools-iris
 

xarray/tutorial.py

@@ -77,8 +77,11 @@ def open_dataset(
     """
     try:
         import pooch
-    except ImportError:
-        raise ImportError("using the tutorial data requires pooch")
+    except ImportError as e:
+        raise ImportError(
+            "tutorial.open_dataset depends on pooch to download and manage datasets."
+            " To proceed please install pooch."
+        ) from e
 
     logger = pooch.get_logger()
     logger.setLevel("WARNING")
@@ -146,8 +149,11 @@ def open_rasterio(
     """
     try:
         import pooch
-    except ImportError:
-        raise ImportError("using the tutorial data requires pooch")
+    except ImportError as e:
+        raise ImportError(
+            "tutorial.open_rasterio depends on pooch to download and manage datasets."
+            " To proceed please install pooch."
+        ) from e
 
     logger = pooch.get_logger()
     logger.setLevel("WARNING")