There is no JavaScript, hence no EJS equivalent, to PHP's exit() or its alias die(). The purpose of both of these PHP functions from the PHP docs:
Output a message and terminate the current script
There is no way to "terminate the current script" in JavaScript. JavaScript always runs to completion with no way to prevent that.
One alternative method of achieving your goal would be to use include and the filename option or, use renderFile() to render this template.
<% if (!something) { %>
<%- include('/notSomethingContent'); %>
<% } else { %>
<%- include('/somethingContent', {something: something}); %>
<% } %>
This would allow you to avoid long indented template content within the else that you "just don't like". :-)