Defer metadata aquisition and application errors during create/extract.

Defer metadata aquisition and application errors during create/extract.

Add MetadataAcquisitionError and MetadataApplicationError.  Throw
MetadataAcquisitionError whenever an attempt to read a path's metadata
fails, and throw MetadataApplicationError whenever an attempt to apply
metadata to a path fails.

Catch and convert these two exceptions to add_error() calls in
Metadata.apply_to_path() and metadata.from_path().  That will defer
them during "bup meta --create" and "bup meta --extract".

Add simple initial tests for the new exceptions.