[MC-31569] Summoning a stacked entity can create a improper error message

/summon Boat ~ ~ ~ {Riding:{id:Villager,Riding:{id:MinecartRideable}}}

This works fine.

/summon SomeFakeEntitiy ~ ~ ~ {Riding:{id:Villager,Riding:{id:MinecartRideable}}}

This gives a logical error message and a command-line warning (no stacktrace), and cancels summoning.

/summon Boat ~ ~ ~ {Riding:{id:Villager,Riding:{id:SomeFakeEntity}}}

This gives no error message error message and a command-line warning (no stacktrace), and summons a entity.

/summon Boat ~ ~ ~ {Riding:{id:SomeFakeEntity,Riding:{id:MinecartRideable}}}

This gives a general error and a stacktrace, but doesn't cancel summoning.
Stacktrace:

Client> 2013-09-18 18:00:16 [SERVER] [WARNING] Skipping Entity with id SomeFakeE
ntity
Client> java.lang.NullPointerException
Client>         at bq.b(SourceFile:88)
Client>         at y.a(SourceFile:55)
Client>         at kt.d(SourceFile:496)
Client>         at kt.a(SourceFile:483)
Client>         at fd.a(SourceFile:47)
Client>         at dz.b(SourceFile:64)
Client>         at kt.e(SourceFile:63)
Client>         at kw.b(SourceFile:37)
Client>         at bnz.b(SourceFile:84)
Client>         at net.minecraft.server.MinecraftServer.u(SourceFile:482)
Client>         at net.minecraft.server.MinecraftServer.t(SourceFile:399)
Client>         at bnw.t(SourceFile:124)
Client>         at net.minecraft.server.MinecraftServer.run(SourceFile:333)
Client>         at ja.run(SourceFile:576)
Client> 2013-09-18 18:00:16 [CLIENT] [INFO] [CHAT] An unknown error occurred whi
le attempting to perform this command