Move exception to it's own file (again)

Moving the exception didn't cause problems in
I925ed62bdc808f0e07862f6e0905e80b50fbe942, but in later changes where
we split blockdevice.py up a bit more, we can get a bit tangled with
circular imports.

Change-Id: I8297483f64c4e1deecd5ec88ee40e9198bb83589

diskimage_builder/block_device/blockdevice.py

@@ -22,6 +22,8 @@ import yaml
 
 from stevedore import extension
 
+from diskimage_builder.block_device.exception import \
+    BlockDeviceSetupException
 from diskimage_builder.block_device.utils import exec_sudo
 from diskimage_builder.graph.digraph import Digraph
 
@@ -29,10 +31,6 @@ from diskimage_builder.graph.digraph import Digraph
 logger = logging.getLogger(__name__)
 
 
-class BlockDeviceSetupException(Exception):
-    pass
-
-
 class BlockDevice(object):
     """Handles block devices.
 

diskimage_builder/block_device/exception.py

@@ -0,0 +1,15 @@
+#  Licensed under the Apache License, Version 2.0 (the "License"); you may
+#  not use this file except in compliance with the License. You may obtain
+#  a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#  License for the specific language governing permissions and limitations
+#  under the License.
+
+
+class BlockDeviceSetupException(Exception):
+    """Generic exception"""

diskimage_builder/block_device/level0/localloop.py

@@ -16,7 +16,7 @@ import logging
 import os
 import subprocess
 
-from diskimage_builder.block_device.blockdevice import \
+from diskimage_builder.block_device.exception import \
     BlockDeviceSetupException
 from diskimage_builder.block_device.tree_config import TreeConfig
 from diskimage_builder.block_device.utils import parse_abs_size_spec

diskimage_builder/block_device/level1/partitioning.py

@@ -17,7 +17,7 @@ import os
 
 from subprocess import CalledProcessError
 
-from diskimage_builder.block_device.blockdevice import \
+from diskimage_builder.block_device.exception import \
     BlockDeviceSetupException
 from diskimage_builder.block_device.level1.mbr import MBR
 from diskimage_builder.block_device.tree_config import TreeConfig

diskimage_builder/block_device/level2/mkfs.py

@@ -15,7 +15,7 @@
 import logging
 import uuid
 
-from diskimage_builder.block_device.blockdevice \
+from diskimage_builder.block_device.exception \
     import BlockDeviceSetupException
 from diskimage_builder.block_device.tree_config import TreeConfig
 from diskimage_builder.block_device.utils import exec_sudo

diskimage_builder/block_device/level3/mount.py

@@ -15,7 +15,7 @@
 import logging
 import os
 
-from diskimage_builder.block_device.blockdevice \
+from diskimage_builder.block_device.exception \
     import BlockDeviceSetupException
 from diskimage_builder.block_device.tree_config import TreeConfig
 from diskimage_builder.block_device.utils import exec_sudo