Helping with broken DETS files.

Apparently something is broken with dets and erlang R16B01. When erlang-history
tries to open the dets file endless {bad_object_header, FileName} are produced.

This commit tries to make the shell usable again without fixing the error. A
warning is issued and infinite crashing is avoided.
addition to commit 7430a90

{bad_object_header, FileName} should be expected everytime when making
dets:lookup
Reinstall the files if possible, just skip backups

The current behaviour stops the install the moment a backup file of
group.erl is found.

However, if the user wants to reinstall multiple times, it might be the
sign of a patch. It is not a problem to install multiple times, only the
original backup of group.erl must be preserved.

This patch moves the install code outside of the backup checking path to
allow for this.