Adjusted API call to cope with cases where not all results are returned in the first query (using the "continue" parameter of the mediawiki API). We were hitting this a handful of times when accessing the full dataset.

Also more informative error reporting when calling the API (list full URL properly), although this is done by altering a reference to the ULR string (yuck).

Also allow get_map_categories() to be called without contacting the toolserver & wmf lab internet sites (quicker, for debugging only)