[lldb] Fix that log enable's -f parameter causes LLDB to crash when it can't open the log file

We didn't do anything with the llvm::Error we get from `Open`, so when we end up in the
error case we just crash due to the llvm::Error sanity check. Also add the missing newline
behind the error message so it no longer messes with the next (lldb) prompt.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D85970