Add error reason to `/tex` for invalid LaTeX

[Zabuzard]

Right now, when using an invalid LaTeX with /tex, it just says that its invalid. But does not give any further reason. The reason is available with the exception in the code though and should be exposed to the user.

Simply change TeXCommand#onSlashCommand to:

} catch (ParseException e) {
    event.reply("That is an invalid latex. Reason: " + e.getMessage())
        .setEphemeral(true)
        .queue();
    return;
}

adding the Reason: " + e.getMessage().