Change message for isUnique method

For the past day I have been using the Check methods and I have found that isUnique method when using strings is not clear when returning the message of error.

```python
from pydeequ.checks import *
from pydeequ.verification import *
import pydeequ
from pyspark.sql.types import DateType, FloatType, StringType, StructField, StructType, BooleanType
import datetime
from pyspark.sql import SparkSession, Row
from pyspark.sql import DataFrame as SparkDataFrame
from typing import Dict, List
import time

mock_orders =[
{
"date": datetime.date(2019, 12, 28),
"country_code": "FR",
"concept_id": "c73bcdcc-2669-4bf6-81d3-e4ae73fb11fd",
"id": "bar",
"gtv": 27.0,
},
{
"date": datetime.date(2019, 12, 20),
"country_code": "UK",
"concept_id": "123e4567-e89b-12d3-a456-426655440000",
"id": "bar",
"gtv": 27.0,
},
]

orders_reference_mock = spark.createDataFrame(data = mock_orders)

check = Check(spark, CheckLevel.Warning, "Review Check")

checkResult = (VerificationSuite(spark)
.onData(orders_reference_mock)
.addCheck(
check
.isUnique("gtv")
.isUnique("id")
)
.run())

checkResult_df = VerificationResult.checkResultsAsDataFrame(spark, checkResult)

checkResult_df.collect()
```
The results is:
```shell
[Row(check='Review Check', check_level='Warning', check_status='Warning', constraint='UniquenessConstraint(Uniqueness(List(gtv),None))', constraint_status='Failure', constraint_message='Value: 0.0 does not meet the constraint requirement!'),
Row(check='Review Check', check_level='Warning', check_status='Warning', constraint='UniquenessConstraint(Uniqueness(List(id),None))', constraint_status='Failure', constraint_message='Value: 0.0 does not meet the constraint requirement!')]
```

The `constraint_message` is not clear and doesn't give any information. It happens in both cases if it is string or integer.

Is it possible to have a more clear message please? I am putting this as a feature instead of a bug

Inizia dal punto di ingresso Check.isUnique e segui il modo in cui VerificationSuite.run produce i risultati per VerificationResult.checkResultsAsDataFrame. Riproduci gli esempi gtv e id, quindi identifica dove viene formato constraint_message. Il lavoro è completato quando i risultati di unicità falliti spiegano quale valore o colonna ha causato il fallimento, invece di riportare soltanto 0.0.

Scritto dal modello di indicizzazione a partire dal testo della issue.