autopkgtest: Don't fail hard on 404 download errors

Dial back commit 570 a bit: We should tolerate 404's, something went wrong on
uploading the result. We still want to fail hard on any other error as these
would indicate infra problems.

autopkgtest.py

@@ -380,7 +380,11 @@ class AutoPackageTest(object):
                 raise NotImplementedError('fetch_one_result(%s): cannot handle HTTP code %i' %
                                           (url, f.getcode()))
         except IOError as e:
-            self.log_error('FATAL: Failure to fetch %s: %s' % (url, str(e)))
+            self.log_error('Failure to fetch %s: %s' % (url, str(e)))
+            # we tolerate "not found" (something went wrong on uploading the
+            # result), but other things indicate infrastructure problems
+            if hasattr(e, 'code') and e.code == 404:
+                return
             sys.exit(1)
 
         try: