Feature/report failed conditions

[amagid]

Description

Closes #1124

Attaching failed validation condition on calls to context.addError. This PR adds a nonenumerable property "condition" to errors resulting from failed validation. This property is nonenumerable and therefore should not affect any existing express APIs that return output from express-validator. However, it can be used to determine which validation condition failed by reading error output and checking the condition property directly. This change maintains express-validator's goal of being unopinionated with regard to error messages while also allowing users to automatically generate human-readable error messages if they want to.

For example, if an input fails an isLength validation, contextError.condition.constructor.name === 'isLength', so I would be able to generate an error message referencing the specific condition that failed rather than a generic message.

To preserve existing functionality with minimal impact, the condition property is always marked optional in the addError function signature.

To-do list

• I have added tests for what I changed.

• This pull request is ready to merge.