cloudinit/importer.py: print a meaningful message if import fails

Sometimes an import might fail for different reasons: the string
is wrongly typed, or the module has a dependency that is not
installed in python.

We should print that there is an import error, otherwise it might be
really difficult to understand what is the root cause of this
issue. Currently, cloud-init just ignores the error and continues.
This can have fatal consequences, especially when used to pick
the datasource to use.

For example, we can have a missing/misspelled datasource file:
$ ls cloudinit/sources/DataSourceInexistent.py
ls: cannot access 'cloudinit/sources/DataSourceInexistent.py': No such file or directory
$ python
>>> __import__("cloudinit.sources.DataSourceInexistent")

and in this case cloud-init should also hint that the error is
"No module named 'cloudinit.sources.DataSourceInexistent'"

If instead the datasource exists but uses a dependency that
is not installed (for example netifaces for DataSourceVMware):
$ ls cloudinit/sources/DataSourceVMWare.py
cloudinit/sources/DataSourceVMWare.py
$ python
>>> __import__("cloudinit.sources.DataSourceVMware")

then cloud-init should hint that the error is
"No module named 'netifaces'"

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>

cloudinit/sources/__init__.py

@@ -929,6 +929,12 @@ def list_sources(cfg_list, depends, pkg_list):
         m_locs, _looked_locs = importer.find_module(
             ds_name, pkg_list, ["get_datasource_list"]
         )
+        if not m_locs:
+            LOG.error(
+                "Could not import %s. Does the DataSource exist and "
+                "is it importable?",
+                ds_name,
+            )
         for m_loc in m_locs:
             mod = importer.import_module(m_loc)
             lister = getattr(mod, "get_datasource_list")