Replaced try-catch statements with assertions

[fearphage]

Replaced try-catch statements with (assert|refute).exception() to make them more explicit about their intent. I also added an ESLint rule to disallow try-catch in the tests directory.

[coveralls]

Coverage remained the same at 94.896% when pulling 9e23c4a on fearphage:remove-try-from-tests into d8dac84 on sinonjs:master.

[lucasfcosta]

Good job! This LGTM.

I also have had a hard time thinking about how to check errors before. I think that by making the whole codebase uniforme we can make it easier for other contributors to find out the right way to do it.