Don't close pipe in SetError (#120)

Fixes #117.

* The pipe shouldn't be closed on SetError function

* Fix TestExecRunsGoWithNoArgsAndGetsUsageMessagePlusErrorExitStatus2 test. It was not checking that exit status is two

* Update test name to match what is really doing

* Fix example for getting exit status from exec

* Add new example for exit status not zero on Exec