Inconsistent error printing at load time

When parsing, we sometimes format as:

"[{}]: Error happened here".format(provenance)

And other times we format as:

"{}: Error happened here".format(provenance)

We should only do the latter, as the provenance will already contain brackets around the column/line, like:

filename.bst [line 0 col 0]: Error happened here

Further, we should make sure to ditch any unnecessary old school "%s" formatting, we prefer "{}".format() across the board.