Model Validator fix and error popup for BECMonitor; closes #56 #39

• Signal validation changed from independent 'name' and 'entry' validation to one model_validator which first validate 'name' and then processed with 'entry'
• New error popup message if there is wrong config with indication where is the problem

Example:

CONFIG_WRONG = {
    "plot_settings": {
        "background_color": "white",
        "num_columns": 5,
        "colormap": "plasma",
        "scan_types": False,
    },
    "plot_data": [
        {
            "plot_name": "BPM4i plots vs samx",
            "x": {"label": "Motor Y", "signals": [{"name": "samx"}]},
            "y": {"label": "bpm4i", "signals": [{"name": "bpm4i"}]},
        },
        {
            "plot_name": "Gauss plots vs samx",
            "x": {"label": "Motor X", "signals": [{"name": "samx"}]},
            "y": {
                "label": "Gauss",
                "signals": [
                    {"name": "gauss_bpm"},
                    {"name": "non_exist1"},
                    {"name": "non_exist2"},
                    {"name": "gauss_bpm", "entry": "samx"},
                ],
            },
        },
    ],
}

Error message: