Replace try-fail-catch pattern with nicer assertThatThrownBy

[iamdanfox]

What happened?

People write:

        try {
            Main.processArgs("-offset", "10", "-length", "20", "-length", "50", "Foo.java");
            fail("fail");
        } catch (UsageException e) {
            // expected
        }

e.g. https://github.com/palantir/palantir-java-format/blob/78e4387d65b232a2fbff3cf9c39fffc7167460ec/palantir-java-format/src/test/java/com/palantir/javaformat/java/CommandLineFlagsTest.java

What did you want to happen?

assertThatThrownBy(() -> Main.processArgs("-offset", "10", "-length", "20", "-length", "50", "Foo.java"))
                .isInstanceOf(UsageException.class);

Stretch goals:

• preserving the message if it's interesting, e.g. fail("hardDeleteObjects should have thrown an exception");
• supporting other types of fail e.g. raw junit fail e.g. org.junit.Assert#fail