Command::execCommand should let sharding StaleConfig exceptions unroll up to receivedQuery

In mongodb version 3.2, as part of the changes to support OP_COMMAND and metadata, we are now catching all exceptions in Command::execCommand. However, this breaks backwards compatibility with older versions of mongos, which expect the ResultFlag_ShardConfigStale bit to be set on the response from the command (which is being run as a query).

Because of this, SendStaleConfigException should just be passed through by the command execution code.

Without this fix we are breaking backwards compatibility with mongos 3.0 for the findAndModify command (and possibly aggregation).