Display clearer indication of KaTeX error

If LaTeX is passed to KaTeX that it cannot process, it now displays in
the preview 'Bad KaTeX: ' followed by the unprocessable source string
with a black border similar to StackExchange.
Remove requirement of blank lines for disp eq

Blank lines are no longer a requirement to preceed and follow a
displayed maths block.
Add support for \[,\] delimiters for display eq

You can infact use either $$, \[ for the opening delimiter and it will
match to either of $$, \], although it is advised to stick to one
particular format in the same document.