Room&Contact.find() should throw exception when it get more than one value

[lijiarui]

When I have two contacts with the same name, using function find() will return one of the contacts randomly, which is not easy to debug.

We should throw an exception when function find() return more than one value.

[huan]

Agree. PR is welcome.

[huan]

Instead of throw, I'm now considering to use a log.warn() instead.

Because sometimes be able to just using the first find result will be very convienence.

[lijiarui]

It's a good idea, I will change then.

[huan]

We can set the level to warn or info. which one do you think is better?

[lijiarui]

I think warn is better, because it can cause more attention.