Raise an exception when the wpr_archive_info isn't created.

This is to provide a more user friendly error. This:

  AssertionError: Pageset archive_data_file path must be specified.

instead of this:

  AttributeError: 'NoneType' object has no attribute 'AddNewTemporaryRecording'

BUG=

Review URL: https://codereview.chromium.org/660143002

Cr-Commit-Position: refs/heads/master@{#300050}

tools/telemetry/telemetry/page/record_wpr.py

@@ -172,6 +172,8 @@ class WprRecorder(object):
     return ps
 
   def Record(self, results):
+    assert self._page_set.wpr_archive_info, (
+      'Pageset archive_data_file path must be specified.')
     self._page_set.wpr_archive_info.AddNewTemporaryRecording()
     self._record_page_test.CustomizeBrowserOptions(self._options)
     page_runner.Run(self._record_page_test, self._page_set,