Sometimes sources file could have invalid format, although that might just happen occasionally. This is useful for downstream client to detect errors, e.g. for a case of rhpkg with exploded sources support.

The error message should be straightforward as much as possible to tell file has invalid content. For example, as suggested by Lubomir, "at the very least the error message for MalformedLineError should actually say that there is a sources file with invalid content."