Add line numbers to lexer errors

Follow up to https://github.com/Shopify/liquid/pull/419

This adds line numbers to lexer errors. There are still many other syntax errors left that don't have line numbers yet, but this PR at least makes it one less :-)

@dylanahsmith @jasonhl for review please
FYI @KnVerey @trishume 
We already have the line number being set in warn mode in Liquid::ParserSwitching, so we should just set the line number in a common place.  Just move [the line](https://github.com/Shopify/liquid/blob/master/lib/liquid/parser_switching.rb#L11) `e.set_line_number_from_token(markup)` from parse_with_selected_parser to strict_parse_with_error_context in Liquid::ParserSwitching, then we don't need any of these changes to Liquid::Parser