Guard against network failures in getDatabaseDate()

When updating use the date from the diff file instead
as we are guaranteed to get this if the file has been
successfully loaded.

utils/osm_file_date.py

@@ -0,0 +1,33 @@
+#!/usr/bin/python
+
+import osmium
+import sys
+import datetime
+
+
+class Datecounter(osmium.SimpleHandler):
+
+    filedate = None
+
+    def date(self, o):
+        ts = o.timestamp
+        if self.filedate is None or ts > self.filedate:
+            self.filedate = ts
+
+    node = date
+    way = date
+    relation = date
+
+
+if __name__ == '__main__':
+    if len(sys.argv) != 2:
+        print("Usage: python osm_file_date.py <osmfile>")
+        sys.exit(-1)
+
+    h = Datecounter()
+
+    h.apply_file(sys.argv[1])
+
+    print(h.filedate)
+
+