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

llvm-svn: 339351