narrow the scope of import error and make sure non - aliases to ansible get their own error

bin/ansible

@@ -71,11 +71,15 @@ if __name__ == '__main__':
                     mycli = getattr(__import__("ansible.cli.%s" % sub, fromlist=[myclass]), myclass)
             elif me == 'ansible':
                 from ansible.cli.adhoc import AdHocCLI as mycli
+            else:
+                raise AnsibleError("Unknown Ansible alias: %s" % me)
+        except ImportError as e:
+            if e.message.endswith(' %s' % sub):
+                raise AnsibleError("Ansible sub-program not implemented: %s" % me)
+            else:
+                raise
 
-            cli = mycli(sys.argv, display=display)
-        except (ImportError,NameError):
-            raise AnsibleError("Ansible sub-program not implemented: %s" % me)
-
+        cli = mycli(sys.argv, display=display)
         cli.parse()
         sys.exit(cli.run())