Add error logging when whitelist lookup fails

Rather than just raising a bare KeyError with no additional information,
this raises a proper KeyError with the key that failed to be looked up.
It also logs the key that failed to load, as well as the whitelist, to
aid in troubleshooting.

salt/loader.py

@@ -1629,7 +1629,11 @@ class LazyLoader(salt.utils.lazy.LazyDict):
                 return True
             # if the modulename isn't in the whitelist, don't bother
             if self.whitelist and mod_name not in self.whitelist:
-                raise KeyError
+                log.error(
+                    'Failed to load key %s because its module (%s) is not in '
+                    'the whitelist: %s', key, mod_name, self.whitelist
+                )
+                raise KeyError(key)
 
             def _inner_load(mod_name):
                 for name in self._iter_files(mod_name):