Skip PresentationManager files we do not support.

openlp/plugins/songs/lib/importers/presentationmanager.py

@@ -54,7 +54,13 @@ class PresentationManagerImport(SongImport):
                 # Open file with detected encoding and remove encoding declaration
                 text = open(file_path, mode='r', encoding=encoding).read()
                 text = re.sub('.+\?>\n', '', text)
+                try:
                     tree = etree.fromstring(text, parser=etree.XMLParser(recover=True))
+                except ValueError:
+                    self.log_error(file_path,
+                                   translate('SongsPlugin.PresentationManagerImport',
+                                             'File is not in XML-format, which is the only format supported.'))
+                    continue
             root = objectify.fromstring(etree.tostring(tree))
             self.process_song(root)