master: Use exception to report errors in fs_loadall()

Add new exception classes:
MetadataException, MetadataConsistencyException,
MetadataFSConsistencyException,
FilesystemException.

Throw MetadataFSConsistencyException if no metadata file were to be found,
throw MetadataConsistencyException if cannot read metadata header,
throw MetadataConsistencyException if wrong metadata version has been found,
throw MetadataConsistencyException on version mismatch,
if backup file is newer than current file,
throw FilesystemException if cannot rename files,
throw MetadataConsistencyException on failed metadata structure read.

Change-Id: I734f2f877db740fd5c66cb19175a354c2cc62859