Make CLI test failures verbose by not catching exceptions in the test runner

[Copilot]

When a CLI test fails, the default FlaskCliRunner swallows exceptions and surfaces only a non-zero exit code, forcing developers to hunt for the root cause manually.

Changes

• flexmeasures/conftest.py: adds CustomFlaskCliRunner — a FlaskCliRunner subclass that defaults catch_exceptions=False — and sets it as test_cli_runner_class on the test app, so all existing app.test_cli_runner() calls get verbose exceptions for free with no per-test changes.

class CustomFlaskCliRunner(FlaskCliRunner):
    def invoke(self, *args, **kwargs):
        kwargs.setdefault("catch_exceptions", False)
        return super().invoke(*args, **kwargs)

• flexmeasures/cli/tests/test_utils.py: adds a strict xfail test that invokes a command raising RuntimeError. It must fail (XFAIL); if exceptions start being silently caught again the test unexpectedly passes (XPASS), breaking the suite.