[PY3 migration]: blink_web_tests - Fix some err_string conversions

Bug: 1285299
Change-Id: I55991bfb46a572484d0b4cab4e2050fa7c696d93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3408181


Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Preethi Mohan <preethim@google.com>
Cr-Commit-Position: refs/heads/main@{#962217}