When running the ios/iossim prepare script show the script output when it...

When running the ios/iossim prepare script show the script output when it returns with a non-zero exit code.

Summary:
Previously we'd just show the exception and not the output from the
executed script. This is unhelpful in the case that the script actually
reports some useful information on the failure.

Now we print the output and re-raise the exception.

Reviewers: kubamracek, george.karpenkov

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D52350

llvm-svn: 342869