improve rotating-file assert on 'count' field

Fixes #207, #208

lib/bunyan.js

@@ -1051,7 +1051,12 @@ function RotatingFileStream(options) {
     this.stream = fs.createWriteStream(this.path,
         {flags: 'a', encoding: 'utf8'});
     this.count = (options.count == null ? 10 : options.count);
-    assert.ok(typeof (this.count) === 'number' && this.count >= 0);
+    assert.equal(typeof (self.count), 'number',
+        format('rotating-file stream "count" is not a number: %j (%s) in %j',
+            self.count, typeof(self.count), self));
+    assert.ok(self.count >= 0,
+        format('rotating-file stream "count" is not >= 0: %j in %j',
+            self.count, self));
 
     // Parse `options.period`.
     if (options.period) {