Improve error in cloud clients when boto3 is missing

Signed-off-by: Marco Nenciarini <marco.nenciarini@2ndquadrant.it>
Signed-off-by: Gabriele Bartolini <gabriele.bartolini@2ndQuadrant.it>

barman/cloud.py

@@ -30,15 +30,18 @@
 from io import BytesIO
 from tempfile import NamedTemporaryFile
 
-import boto3
-from botocore.exceptions import ClientError, EndpointConnectionError
-
 from barman.backup_executor import ConcurrentBackupStrategy
 from barman.fs import path_allowed
 from barman.infofile import BackupInfo
 from barman.postgres_plumbing import EXCLUDE_LIST, PGDATA_EXCLUDE_LIST
 from barman.utils import force_str, human_readable_timedelta, total_seconds
 
+try:
+    import boto3
+    from botocore.exceptions import ClientError, EndpointConnectionError
+except ImportError:
+    raise SystemExit("Missing required python module: boto3")
+
 try:
     # Python 3.x
     from urllib.parse import urlparse