Propagate command interpreter errors from lldlbinit

This patch ensures that we propagate errors coming from the lldbinit
file trough the command/script interpreter. Before, if you did something
like command script import syntax_error.py, and the python file
contained a syntax error, lldb wouldn't tell you about it. This changes
with the current patch: errors are now propagated by default.

PS: Jim authored this change and I added testing.

Differential revision: https://reviews.llvm.org/D61579

llvm-svn: 360216