[CommandObjectCommands] Honor stop-command-source-on-error

This patch ensures that we honor the stop-command-source-on-error
setting from `command source`. The problem is that we didn't
differentiate between the boolean value being true or false, or not
being set. For the latter scenario, we should calculate the value in the
command interpreter based on the global options.

Differential revision: https://reviews.llvm.org/D61406

llvm-svn: 359750