Better formatting for error messages

[phackstock]

Some error messages are currently somewhat hard to read, example:

pydantic.error_wrappers.ValidationError: 1 validation error for RegionProcessor
mappings -> ['REMIND 3.0']
  Region(s) ['REMIND 3.0|Canada, NZ, Australia', 'REMIND 3.0|China', 'REMIND 3.0|EU 28', 'REMIND 3.0|India', 'REMIND 3.0|Japan', 'REMIND 3.0|Latin America and the Caribbean', 'REMIND 3.0|Middle East, North Africa, Central Asia', 'REMIND 3.0|Non-EU28 Europe', 'REMIND 3.0|other Asia', 'REMIND 3.0|Countries from the Reforming Economies of the Former Soviet Union', 'REMIND 3.0|Sub-saharan Africa', 'REMIND 3.0|United States of America'] in mappings/REMIND+3.0.yml not defined in the DataStructureDefinition (type=value_error.region_not_defined; region=['REMIND 3.0|Canada, NZ, Australia', 'REMIND 3.0|China', 'REMIND 3.0|EU 28', 'REMIND 3.0|India', 'REMIND 3.0|Japan', 'REMIND 3.0|Latin America and the Caribbean', 'REMIND 3.0|Middle East, North Africa, Central Asia', 'REMIND 3.0|Non-EU28 Europe', 'REMIND 3.0|other Asia', 'REMIND 3.0|Countries from the Reforming Economies of the Former Soviet Union', 'REMIND 3.0|Sub-saharan Africa', 'REMIND 3.0|United States of America']; file=mappings/REMIND+3.0.yml)

simply introducing a few strategically placed line breaks should make things better:

pydantic.error_wrappers.ValidationError: 1 validation error for RegionProcessor
mappings -> ['REMIND 3.0']
Region(s):
 ['REMIND 3.0|Canada, NZ, Australia', 'REMIND 3.0|China', 'REMIND 3.0|EU 28', 'REMIND 3.0|India', 'REMIND 3.0|Japan', 'REMIND 3.0|Latin America and the Caribbean', 'REMIND 3.0|Middle East, North Africa, Central Asia', 'REMIND 3.0|Non-EU28 Europe', 'REMIND 3.0|other Asia', 'REMIND 3.0|Countries from the Reforming Economies of the Former Soviet Union', 'REMIND 3.0|Sub-saharan Africa', 'REMIND 3.0|United States of America'] 
    in mappings/REMIND+3.0.yml 
not defined in the DataStructureDefinition 
(type=value_error.region_not_defined; region=['REMIND 3.0|Canada, NZ, Australia', 'REMIND 3.0|China', 'REMIND 3.0|EU 28', 'REMIND 3.0|India', 'REMIND 3.0|Japan', 'REMIND 3.0|Latin America and the Caribbean', 'REMIND 3.0|Middle East, North Africa, Central Asia', 'REMIND 3.0|Non-EU28 Europe', 'REMIND 3.0|other Asia', 'REMIND 3.0|Countries from the Reforming Economies of the Former Soviet Union', 'REMIND 3.0|Sub-saharan Africa', 'REMIND 3.0|United States of America']; file=mappings/REMIND+3.0.yml)

or something along those lines.