Minor shuffling of installd error reporting.

We actually want to early-return when creating user data fails, and
we want to return an exception when getAppDataInode() is called with
unsupported flags.

Test: builds, boots, apps install/uninstall fine
Bug: 13758960, 30944031
Change-Id: I69816275a0be7a73a8ae7f3d90f2cdea80d8ebfc