Don't crash when wiping backup data redundantly in the local transport

Previously, if using the "local" debugging transport:

    adb shell bmgr wipe com.android.browser
    adb shell bmgr wipe com.android.browser

... would bring down the runtime.  This no longer happens.  The fix
covers two aspects of the situation:  1. the local transport no longer
blows up in this use case, and 2. the backup manager itself now catches
blowups on the part of the transport, and tidies up after them.

Bug 6205185

Change-Id: Ieb9b8827a62523148ad5a0ec15b05a954d198b3d