Add more error handling

[pe-st]

• when DEVICE_DETAIL is empty/None
• when ACTIVITY_DETAIL gets HTTP error 500
• when a['activityName'] is None

This should also eliminate the problem of #11.

Error output for the missing DEVICE_DETAIL:

Garmin Connect activity: [2487420533] Some Running
https://connect.garmin.com/modern/proxy/download-service/export/gpx/activity/2487420533?full=true
    Downloading file... Activity summary URL: https://connect.garmin.com/modern/proxy/activity-service/activity/2487420533
Device detail URL: https://connect.garmin.com/modern/proxy/device-service/deviceservice/app-info/799988
Writing empty file since there was no GPX activity data...
Traceback (most recent call last):
  File "./gcexport3.py", line 477, in <module>
    DEVICE_DETAIL.decode(),
AttributeError: 'str' object has no attribute 'decode'

Error Output for HTTP error 500 for ACTIVITY_DETAIL:

Garmin Connect activity: [2693805666] Some Running
https://connect.garmin.com/modern/proxy/download-service/export/gpx/activity/2693805666?full=true
    Downloading file... Activity summary URL: https://connect.garmin.com/modern/proxy/activity-service/activity/2693805666
Device detail URL: https://connect.garmin.com/modern/proxy/device-service/deviceservice/app-info/845288
Activity details URL: https://connect.garmin.com/modern/proxy/activity-service-1.3/json/activityDetails/2693805666
Traceback (most recent call last):
  File "./gcexport3.py", line 484, in <module>
    ACTIVITY_DETAIL = http_req(URL_GC_ACTIVITY_DETAIL + str(a["activityId"]))
  File "./gcexport3.py", line 151, in http_req
    response = OPENER.open((request), data=post)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 531, in open
    response = meth(req, response)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 641, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 569, in error
    return self._call_chain(*args)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 503, in _call_chain
    result = func(*args)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 649, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 500: Internal Server Error

Error Output for missing ActivityName:

Garmin Connect activity: [630980785] None
https://connect.garmin.com/modern/proxy/download-service/export/gpx/activity/630980785?full=true
    Downloading file... Activity summary URL: https://connect.garmin.com/modern/proxy/activity-service/activity/630980785
Device detail URL: https://connect.garmin.com/modern/proxy/device-service/deviceservice/app-info/720039
Activity details URL: https://connect.garmin.com/modern/proxy/activity-service-1.3/json/activityDetails/630980785
Traceback (most recent call last):
  File "./gcexport3.py", line 508, in <module>
    else '"' + a["activityName"].replace('"', '""') + '",'
AttributeError: 'NoneType' object has no attribute 'replace'

[moderation]

Thanks for the updates @pe-st. Just tested and looking good. /cc @tobstarr