Error code for test timeout varies between browsers

[andydavies]

The agent generates error codes for failures e.g. navigation failed, timeouts etc.

devtools.py uses 99997 when a test times out

wptagent/internal/devtools.py

Line 805 in e7ad586

self.task['page_data']['result'] = 99997

but other browser e.g. firefox.py, uses 99998

wptagent/internal/firefox.py

Line 412 in e7ad586

self.task['page_data']['result'] = 99998

Why is the code different?
Should we make them consistent?