Some test don't provide error message (they can fail for other thing)

[A-312]

In my PR #342, I saw some tests which don't provide error message like this : .must.not.throw(); Then they can failed for other thing and not for format validate. (Like undefined variable in npm convict code...) instead of : 'must be a prime number: value was 16'.

node-convict/test/format-tests.js

Line 161 in d263763

 (function() { conf.validate(); }).must.throw();