[era_check] Was returning after just checking the superblock.

Presumably old debug.
[block_manager] Track the number of superblocks, throw an exception if someone tries to open two a once.
Update gemfile.lock
[era/metadata.{h,cc}] Support creation of new metadata
[era/xml format] bring in line with the Ruby library
[era/xml restorer] bug fixes
[btree_damage_tracker] Initialise all path_tracker objects with a null path.

This fixes an insidious bug that took me ages to track down.  Without
it the non-existent front() of a list was sometimes being accessed.
[era_restore] add some cucumber tests