tests: fix assert.throws() usage

[stephenplusplus]

So, I was using mocha wrong in regards to testing thrown errors.

Wrong:

assert.throw(function() {
  new Bucket();
}, 'You need a name for this bucket.'); // <-- not the real error message

Right:

assert.throw(function() {
  new Bucket();
}, /You need a name for this bucket\./); // <-- not the real error message

[coveralls]

Coverage remained the same at 100.0% when pulling 5c4191a on stephenplusplus:spp--throwing-regex into 143f120 on GoogleCloudPlatform:master.

[stephenplusplus]

@callmehiphop how's this one look?

[callmehiphop]

LGTM!