Fix error message formatting in _handle_proxies and grammatical error

[JosephPTM1]

Python doesn't allow "%s", in exceptions, this will raise the exception as a tuple with "Invalid JSON passed for argument 'proxies': %s" as a literal string and proxies as the other element in the tuple. I have changed it to an f string in this case rather than adapting it to "%s" % proxies because TM1py is targeting Python 3.7 and f strings are faster and more readable. I tested this and it does indeed just raise that tuple.

That being said, feel free to change this to "%s" % proxies if the consistency of it is important.

I also spotted a minor grammatical error while reading through the rest of the code with the line Must be to be of type 'bool' or 'str'".

[JosephPTM1]

Apologies for raising 2 PRs in what could have been 1. The first was a fix for something I had just personally experienced, this one was through just reading the rest of the code out of intrigue.