Also display the output and error output of a failed command
Also display the output and error output of a failed command

Summary:
Instead of just printing the current "False is not True, ..." message when we
fail to run a certain command, this patch also adds the actual command output or
error output that we received to the assertion message.

Reviewers: davide

Reviewed By: davide

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D50492

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@339351 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 3f0e16cfe858eea5010f70606d2ba9a170263bf0)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb