[test] Report error when inferior test processes exit with a non-zero code

Summary:
We've run into this problem when the test errored out so early (because it could not connect to
the remote device), that the code in D20193 did not catch the error. This resulted in the test
suite reporting success with 0 tests being run.

This patch makes sure that any non-zero exit code from the inferior process gets reported as an
error. Basically I expand the concept of "exceptional exits", which was previously being used for
signals to cover these cases as well.

Reviewers: tfiala, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D22404

llvm-svn: 275782