Remove MetadataError and make apply error handling finer-grained.

Remove MetadataError and make apply error handling finer-grained.

Handle errors that were handled by a blanket MetadataApplyError catch
more selectively.  Rename MetadataApplyError to ApplyError.  Throw an
ApplyError when encountering EACCES in order to skip the (bad) path.
Call add_error() for lchown EPERM.  Allow other exceptions to pass
through.

Catch ApplyError when calling apply_to_path() from extract() and
finish_extract() and redirect the exception to add_error() --
i.e. skip the path.

Add bup.helpers.clear_errors(), and use it in test cases that involve
calls to add_error().