Use correct exception for boto3

We use boto3 for our interaction with s3. Therefore if an expection is
thrown it will be thrown from the botocore library (which boto3 is built
on top of).

I have copied
app/aws/s3.py::file_exists for an example of this exception catching.

app/dao/notifications_dao.py

@@ -6,7 +6,7 @@ from datetime import (
     timedelta,
 )
 
-from boto.exception import BotoClientError
+from botocore.exceptions import ClientError
 from flask import current_app
 from notifications_utils.international_billing_rates import INTERNATIONAL_BILLING_RATES
 from notifications_utils.recipients import (
@@ -438,7 +438,7 @@ def _delete_letters_from_s3(
             for s3_object in s3_objects:
                 try:
                     remove_s3_object(bucket_name, s3_object['Key'])
-                except BotoClientError:
+                except ClientError:
                     current_app.logger.exception(
                         "Could not delete S3 object with filename: {}".format(s3_object['Key']))