fix #537 ignore waypoint markers with invalid

coordinates on import.

I've also added a unit test with such an export file.

src/main/java/de/dennisguse/opentracks/io/file/importer/AbstractFileTrackImporter.java

@@ -301,7 +301,8 @@ abstract class AbstractFileTrackImporter extends DefaultHandler implements Track
         TrackPoint trackPoint = createTrackPoint();
 
         if (!LocationUtils.isValidLocation(trackPoint.getLocation())) {
-            throw new SAXException(createErrorMessage("Invalid location detected: " + trackPoint));
+            Log.w(TAG, "Marker with invalid coordinates ignored: " + trackPoint.getLocation());
+            return;
         }
         Marker marker = new Marker(trackPoint.getLocation());