Better handling of parameters errors

[lucagiove]

Ohhhh there was a missing space before expected_status=any in

Patch On Session blockfilter ${path} json=${data} headers=${headers} expected_status=any

It would be cool to have a better error message than AttributeError: 'str' object has no attribute 'items'

The call stack is below if it can help to have a better message:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/RequestsLibrary/utils.py", line 138, in decorator
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/RequestsLibrary/RequestsOnSessionKeywords.py", line 106, in patch_on_session
    response = self._common_request("patch", session, url,
  File "/usr/local/lib/python3.9/site-packages/RequestsLibrary/SessionKeywords.py", line 569, in _common_request
    resp = method_function(
  File "/usr/local/lib/python3.9/site-packages/requests/sessions.py", line 602, in patch
    return self.request('PATCH', url, data=data, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/requests/sessions.py", line 516, in request
    prep = self.prepare_request(req)
  File "/usr/local/lib/python3.9/site-packages/requests/sessions.py", line 449, in prepare_request
    p.prepare(
  File "/usr/local/lib/python3.9/site-packages/requests/models.py", line 315, in prepare
    self.prepare_headers(headers)
  File "/usr/local/lib/python3.9/site-packages/requests/models.py", line 447, in prepare_headers
    for header in headers.items():

Originally posted by @dprevost-LMI in https://github.com/MarketSquare/robotframework-requests/issues/314#issuecomment-778517767