typechecker: Improve error messages for casting.

Summary: Raise semantic errors for object casts, instead of parse errors

Reviewed By: @elgenie

Differential Revision: D1513322
flakeylist socket_create_listen

Summary: This failed 8 times yesterday after my rand fix. Lets just kill it since tests without retries and binding are horrible.

Reviewed By: @fredemmott

Differential Revision: D1520240