General validation function.
Validates that the files exist.
Validates that the file has one of the allowed extensions.
Validates that the file is writable.
Validates that the files exist.
Validates that value is positive.
Validates that the values described by rangeString are non-negative and in ascending order. Validation is skipped if rangeString is null.
Basic format of validation error messages.
Validate and parse config files.