Remove MetadataAcquireError and make error handling finer-grained

Remove MetadataAcquireError and make error handling finer-grained

Handle errors that were handled by a blanket MetadataAcquireError
catch more selectively.  Call add_error() instead for any OSError
exception from readlink(), and for any EnvironmentError exception from
get_linux_file_attr() where errno == EACCES (i.e. chmod 000).  Allow
other exceptions to pass through.