show error when parsing with KaTeX

[arrocke]

• Use KaTeX's built in error reporting instead of throwing an exception and logging to the console.
• For KaTeX Parse error #1737

I ran into one issue and I'm not sure what standard procedure is to address since is not directly a part of this proposed change. These lines in EmbedBlot are throwing an error when I insert any formula, valid or invalid. I took look at git blame and found that this.contentNode was introduced in the last two weeks, but this is undefined because the function has a different context than the constructor. If that function is changed to an arrow function or .bind(this) is added to the end, that fixes the issue.

[jhchen]

Thanks!