Improve error message on invalid user_config_json

When the parameter value was not a valid json, the error message was empty.

    ERROR	command failed: UserError:

Add a detailed error message showing the location of the error:

    ERROR	command failed: UserError: Invalid user_config_json: Expecting value: line 8 column 13 (char 202)

aiven/client/cliarg.py

@@ -70,7 +70,7 @@ def wrapped(self: CommandLineTool) -> T:
                     get_json_config(self.args.user_config_json),
                 )
             except jsonlib.decoder.JSONDecodeError as err:
-                raise UserError from err
+                raise UserError(f"Invalid user_config_json: {err!s}") from err
             return fun(self)
 
         return wrapped

tests/test_cliarg.py

@@ -4,11 +4,12 @@
 # See the file `LICENSE` for details.
 
 # pylint: disable=no-member
+from _pytest.logging import LogCaptureFixture
 from aiven.client.argx import CommandLineTool
 from aiven.client.cliarg import arg  # type: ignore
 
 
-def test_user_config_json_error_json() -> None:
+def test_user_config_json_error_json(caplog: LogCaptureFixture) -> None:
     """Test that @arg.user_config_json causes
     CommandLineTool.run() to exit cleanly with return value 1
     if JSON is incorrect
@@ -22,9 +23,14 @@ class T(CommandLineTool):
         def t(self) -> None:
             """t"""
 
-    error_json_arg = ["t", "--user-config-json", "foo"]
+    error_json_arg = ["t", "--user-config-json", "bar"]
     test_class = T("avn")
     ret = test_class.run(args=error_json_arg)
+    # The message can vary across python versions
+    assert (
+        "Invalid user_config_json: Expecting value: line 1 column 1 (char 0)" in caplog.text
+        or "Invalid user_config_json: Unexpected 'b': line 1 column 1 (char 0)" in caplog.text
+    )
     assert ret == 1