Add check for validate_data consistency to validate-project CLI

[danielhuppmann]

This PR extends the validate-project CLI to check the consistency of all yaml files in the validate_data folder.

The error output is

Error(s) in files for DataValidator:
Collected 2 errors:
  1. In file data/validation/validate_data/validate_unknown_region.yaml:
  Collected 1 error:
    1. The following regions are not defined in the DataStructureDefinition:
       Asia

  2. In file data/validation/validate_data/validate_unknown_variable.yaml:
  Collected 1 error:
    1. The following variables are not defined in the DataStructureDefinition:
       Final Energy|Industry

Could be extended to clearly state the folder/type that is being checked in the first row...

[danielhuppmann]

Reworked the error log message to be more concise:

Collected 2 errors when checking files for 'DataValidator' (validation/validate_data):
  1. Collected 1 error when checking in file 'validation/validate_data/validate_unknown_region.yaml':
    1. The following regions are not defined in the DataStructureDefinition:
       Asia

  2. Collected 1 error when checking in file 'validation/validate_data/validate_unknown_variable.yaml':
    1. The following variables are not defined in the DataStructureDefinition:
       Final Energy|Industry

[phackstock]

Looks good to me, one small comment in line, after that good to merge