query: add --warn-exceptions, dateparser, docs

[purarue]

added --warn-exceptions (like --raise-exceptions/--drop-exceptions, but lets you pass a warn_func if you want to customize how the exceptions are handled. By default this creates a logger in main and logs the exception

added dateparser as a fallback if its installed (it's not a strong dependency, but I mentioned in the docs that it's useful for parsing dates/times)

added docs for query, and a few examples

--output gpx respects the --{drop,warn,raise}--exceptions flags, have an example of that in the docs as well

[purarue]

Oh also, moved some of the error iterators drop_exeptions,raise_exceptions from query to my.core.error, I find myself using those pretty often so I think would be useful to make those 'public'

[purarue]

Alright, have read over this a few times, should be good to review

[karlicoss]

thanks! great docs too!