Remove unused exceptions

* nova/exception (ImageDownloadModuleLoadError):
Never referenced.
* nova/exception (ISERTargetNotFoundForVolume):
Never referenced.
* nova/exception (InstanceMetadataNotFound):
Unreferenced since commit 0d417fa73
* nova/exception (ISCSITargetNotFoundForVolume):
Unreferenced since commit 4123094b2

This commit removes them.

Change-Id: I224c6b1d711add12320ca8e89486257a6517a1c9
Merge "Remove unused exceptions"