daklib/checks.py: give a nice error for sources with invalid versions

Give a proper error message if a source package includes an upstream
tarball, but doesn't have a Debian revision.

daklib/checks.py

@@ -375,7 +375,10 @@ class SourceCheck(Check):
 
         version = control['Version']
         if is_orig:
-            version = re_field_version_upstream.match(version).group('upstream')
+            upstream_match = re_field_version_upstream.match(version)
+            if not upstream_match:
+                raise Reject('{0}: Source package includes upstream tarball, but {0} has no Debian revision.'.format(filename, version))
+            version = upstream_match.group('upstream')
         version_match =  re_field_version.match(version)
         version_without_epoch = version_match.group('without_epoch')
         if match.group('version') != version_without_epoch: