Improve required data output

[phackstock]

Currently the output of the RequiredDataValidator in the case of missing variables is not useful in understanding what exactly is missing. For example:

2023-06-29 13:49:37 ERROR    Required data [{'region': ['ABW', 'AFG', 'AGO', 'ALB', 'ARE', 'ARG', 'ARM', 'ASM', 'ATG', 'AUS', 'AUT', 'AZE', 'BDI', 'BEL', 'BEN', 'BFA', 'BGD', 'BGR', 'BHR', 'BHS', 'BIH', 'BLR', 'BLZ', 'BMU', 'BOL', 'BRA', 'BRB', 'BRN', 'BTN', 'BWA', 'CAF', 'CAN', 'CHE', 'CHL', 'CHN', 'CIV', 'CMR', 'COD', 'COG', 'COK', 'COL', 'COM', 'CPV', 'CRI', 'CUB', 'CUW', 'CYM', 'CYP', 'CZE', 'DEU', 'DJI', 'DMA', 'DNK', 'DOM', 'DZA', 'ECU', 'EGY', 'ERI', 'ESH', 'ESP', 'EST', 'ETH', 'EU27', 'FIN', 'FJI', 'FLK', 'FRA', 'FRO', 'FSM', 'GAB', 'GBR', 'GEO', 'GHA', 'GIB', 'GIN', 'GLP', 'GMB', 'GNB', 'GNQ', 'GRC', 'GRD', 'GRL', 'GTM', 'GUF', 'GUM', 'GUY', 'HKG', 'HND', 'HRV', 'HTI', 'HUN', 'IDN', 'IND', 'IRL', 'IRN', 'IRQ', 'ISL', 'ISR', 'ITA', 'JAM', 'JOR', 'JPN', 'KAZ', 'KEN', 'KGZ', 'KHM', 'KIR', 'KNA', 'KOR', 'KWT', 'LAO', 'LBN', 'LBR', 'LBY', 'LCA', 'LIE', 'LKA', 'LSO', 'LTU', 'LUX', 'LVA', 'MAC', 'MAR', 'MDA', 'MDG', 'MDV', 'MEX', 'MHL', 'MKD', 'MLI', 'MLT', 'MMR', 'MNE', 'MNG', 'MOZ', 'MRT', 'MSR', 'MTQ', 'MUS', 'MWI', 'MYS', 'NAM', 'NCL', 'NER', 'NGA', 'NIC', 'NIU', 'NLD', 'NOR', 'NPL', 'NZL', 'OMN', 'PAK', 'PAN', 'PER', 'PHL', 'PLW', 'PNG', 'POL', 'PRI', 'PRK', 'PRT', 'PRY', 'PSE', 'PYF', 'QAT', 'REU', 'ROU', 'RUS', 'RWA', 'SAU', 'SDN', 'SEN', 'SGP', 'SLB', 'SLE', 'SLV', 'SOM', 'SPM', 'SRB', 'SSD', 'STP', 'SUR', 'SVK', 'SVN', 'SWE', 'SWZ', 'SXM', 'SYC', 'SYR', 'TCA', 'TCD', 'TGO', 'THA', 'TJK', 'TKL', 'TKM', 'TLS', 'TON', 'TTO', 'TUN', 'TUR', 'TWN', 'TZA', 'UGA', 'UKR', 'URY', 'USA', 'UZB', 'VCT', 'VEN', 'VGB', 'VIR', 'VNM', 'VUT', 'WLF', 'WSM', 'YEM', 'ZAF', 'ZMB', 'ZWE'], 'year': [2010, 2015, 2020, 2025, 2030, 2035, 2040, 2045, 2050], 'variable': 'Final Energy|Residential and Commercial|Electricity', 'unit': 'EJ/yr'}] from file applications/ngfs-phase-4-internal-workflow/required_data/Downscaling_requirements.yaml missing for:
                        model   scenario
0  Downscaling[GCAM 6.0 NGFS]  d_delfrag
1  Downscaling[GCAM 6.0 NGFS]   d_strain
2  Downscaling[GCAM 6.0 NGFS]     h_cpol
3  Downscaling[GCAM 6.0 NGFS]      h_ndc
4  Downscaling[GCAM 6.0 NGFS]     o_1p5c
5  Downscaling[GCAM 6.0 NGFS]       o_2c
6  Downscaling[GCAM 6.0 NGFS]   o_lowdem

does not tell us a lot in terms of region and year where the data is missing.
This needs to be improved to give better feedback.