util/cexec: Implement exception helpers

See src/stdplus/util/cexec.hpp callCheckErrno for the motivation behind
this change.

Change-Id: I0225b87398b632624f2ef8ccd6c00b5dd6b7e056
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.gitignore b/.gitignore
index f58f9f5..b7f492b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -44,3 +44,4 @@
 /test/handle/copyable
 /test/handle/managed
 /test/signal
+/test/util/cexec