added some error messages

src/y.js

@@ -92,13 +92,21 @@ function Y (opts/* :YOptions */) /* :Promise<YConfig> */ {
   }
   Y.sourceDir = opts.sourceDir
   return Y.requestModules(modules).then(function () {
-    return new Promise(function (resolve) {
-      var yconfig = new YConfig(opts)
-      yconfig.db.whenUserIdSet(function () {
-        yconfig.init(function () {
-          resolve(yconfig)
+    return new Promise(function (resolve, reject) {
+      if (opts == null) reject('An options object is expected! ')
+      else if (opts.connector == null) reject('You must specify a connector! (missing connector property)')
+      else if (opts.connector.name == null) reject('You must specify connector name! (missing connector.name property)')
+      else if (opts.db == null) reject('You must specify a database! (missing db property)')
+      else if (opts.connector.name == null) reject('You must specify db name! (missing db.name property)')
+      else if (opts.share == null) reject('You must specify a set of shared types!')
+      else {
+        var yconfig = new YConfig(opts)
+        yconfig.db.whenUserIdSet(function () {
+          yconfig.init(function () {
+            resolve(yconfig)
+          })
         })
-      })
+      }
     })
   })
 }