Fail gracefully if the blivet module can not be imported.

XXX the error messages can be be misleading, as we first try to import
blivet3 and then blivet, so one will see the error from the blivet import
even when then one from blivet3 wold be more relevant (on RHEL 7).

library/blivet.py

@@ -77,6 +77,9 @@
 '''
 
 import logging
+import traceback
+
+BLIVET_PACKAGE = None
 
 try:
     from blivet3 import Blivet
@@ -88,20 +91,25 @@
     from blivet3.util import set_up_logging
     BLIVET_PACKAGE = 'blivet3'
 except ImportError:
-    from blivet import Blivet
-    from blivet.callbacks import callbacks
-    from blivet.flags import flags as blivet_flags
-    from blivet.formats import get_format
-    from blivet.partitioning import do_partitioning
-    from blivet.size import Size
-    from blivet.util import set_up_logging
-    BLIVET_PACKAGE = 'blivet'
-
-from ansible.module_utils.basic import AnsibleModule
-
-blivet_flags.debug = True
-set_up_logging()
-log = logging.getLogger(BLIVET_PACKAGE + ".ansible")
+    try:
+        from blivet import Blivet
+        from blivet.callbacks import callbacks
+        from blivet.flags import flags as blivet_flags
+        from blivet.formats import get_format
+        from blivet.partitioning import do_partitioning
+        from blivet.size import Size
+        from blivet.util import set_up_logging
+        BLIVET_PACKAGE = 'blivet'
+    except ImportError:
+        LIB_IMP_ERR = traceback.format_exc()
+
+from ansible.module_utils.basic import AnsibleModule, missing_required_lib
+
+if BLIVET_PACKAGE:
+    blivet_flags.debug = True
+    set_up_logging()
+    log = logging.getLogger(BLIVET_PACKAGE + ".ansible")
+
 use_partitions = None  # create partitions on pool backing device disks?
 disklabel_type = None  # user-specified disklabel type
 
@@ -636,6 +644,9 @@ def run_module():
 
     module = AnsibleModule(argument_spec=module_args,
                            supports_check_mode=True)
+    if not BLIVET_PACKAGE:
+        module.fail_json(msg=missing_required_lib("blivet"),
+                         exception=LIB_IMP_ERR)
 
     if not module.params['pools'] and not module.params['volumes']:
         module.exit_json(**result)