rename holland.core.backup.error -> holland.core.backup.exc

- consistency with holland.core
make BackupError inherit from HollandError
fix references to holland.core.backup.error
add initial catalog database support
update comments in models.py
add missing docstring
add replace_symlink to holland.core.util.path

Atomically replaces a symlink with a new name. Used by
the new backup api when rotating backups
Merge branch 'amsterdam' into refactor_backup_api
add missing imports
Merge branch 'cli_cleanups' into amsterdam
Merge branch 'amsterdam' into refactor_backup_api
remove deprecated hooks option in [holland:backup]
cleaning plugin loader logging

- bumped several verbose messages to debug mode
Merge branch 'refactor_core_plugin_api' into amsterdam

Conflicts:
	holland/core/plugin/loader.py
Merge branch 'amsterdam' into refactor_backup_api
refactor backup api