Don't let bmgr leave a restore session hanging on error

Specifically, don't wait for the RestoreObserver to be informed that the restore
has completed unless performRestore() ran.  We were winding up in a case where
bmgr was hanging forever waiting on a nonexistent restore process instead of
calling endRestoreSession().

Also improve the documentation, explicitly calling out the need to call
endRestoreSession() even if previous operations on the session were
unsuccessful.