Handle JSON API errors

[juangallostra]

Supersedes #52

Description of the issue/feature this PR addresses

Linked issue: #51

Current behavior before PR

When a query to the JSON API for fetching data is not successful (either an error response is received or no response is received at all) then getting the value of the key count from the response raises an error:

senaite.sync/src/senaite/sync/updatestep.py

Line 74 in bee2388

number_of_pages = (cd["count"]/effective_window) + 1

senaite.sync/src/senaite/sync/fetchstep.py

Line 122 in d029f39

number_of_pages = (cd["count"]/effective_window) + 1

Error message:

We’re sorry, but there seems to be an error…

Here is the full error message:

Display traceback as text

Traceback (innermost last):

     Module ZPublisher.Publish, line 138, in publish
     Module ZPublisher.mapply, line 77, in mapply
     Module ZPublisher.Publish, line 48, in call_object
     Module senaite.sync.browser.views, line 95, in __call__
     Module senaite.sync.updatestep, line 34, in run
     Module senaite.sync.updatestep, line 68, in _fetch_data

KeyError: 'count' 

Desired behavior after PR is merged

When receiveing an error response or no response at all log the error and return without raising an exception. Note that we are logging the response error message. We can be sure that if the API response contains an error message the key message will exist due to: https://github.com/senaite/senaite.jsonapi/blob/871959f4b1c9edbb477e9456325527ca78e13ec6/src/senaite/jsonapi/exceptions.py#L11

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.

[xispa]

No need of that redundancy. The following should be enough:

error_message = "Error message: {}".format(cd.get('message', None) or '')