feat: allow specifying error message override for duplicate key errors unique: true

[vkarpov15]

Summary

Allow overriding MongoDB's default "duplicate key error" message in your schemas:

new Schema({
  email: { type: String, unique: [true, 'Email must be unique'] }
});

This works for save() and query methods, but does not work for bulkWrite() and insertMany() because the duplicate key error in that case does not return keyPattern, which means we would either have to guess the duplicate key by the index name (brittle) or parse the index key from the error message (we would have to pull in Acorn or something like that, because duplicate key error message is not formatted as JSON).

Examples

[hasezoey]

LGTM, minor docs things

[AbdelrahmanHafez]

LGTM