Provide better error when pointed to something else than a Huawei LTE device

[scop]

$ python x.py 
Traceback (most recent call last):
  File "x.py", line 8, in <module>
    connection = Connection('http://httpbin.org/')
  File "[...]/huawei_lte_api/Connection.py", line 45, in __init__
    self._initialize_csrf_tokens_and_session()
[...]
xml.parsers.expat.ExpatError: syntax error: line 1, column 54

Should provide a more indicative error than a generic XML parse one. Preferably something that the user code can examine to determine that the server is not a Huawei LTE device (e.g. somehow the 404 passed through). Perhaps should not try parsing XML for 404's, use requests' raise_for_error selectively (or everywhere except selectively not) etc.

[Salamek]

There is no 100% way to check if device we are trying to connect is a Huawei LTE modem (except checking MAC but that is BS)...
So i think best way will be to handle that XML parse exception and "rethrow" our own IDK something like:

raise ResponseParseException('Failed to parse XML response from device, are you sure that {host} is a supported Huawei LTE Modem?'.format(host=host))

@scop What do you think?

[scop]

As mentioned, I think the HTTP status code should be a part of the exception thrown there, so users can use that in their decision making. Possibly also some other sanity checks on the structure we get if the XML parses successfully, if doable.

In case, let's chain the original exception if there is one, like raise OurException(..., htttp_status=...) from originalexception

[Salamek]

Related #122