TaskUtil: add extra comments
error cleanly if TEST_HOST is unset for app tests

Summary:
when running app tests, look at TEST_HOST. If it's unset, fail fast with
a (hopefully) informative error, rather than dying with an objc
exception (OSX) or mysteriously running 0 tests (iOS.) Add test cases
for valid/invalid TEST_HOST. (Also, add a bit to one of the sample
projects, and some gratuitous comments elsewhere.)

Test Plan: some tests included. Also, ran xctool from shell in
TestProject-App-OSX with TEST_HOST set correctly (expected behavior: 2/3
of the sample project's tests pass) and incorrectly (expected: xctool
prints "TEST_HOST not executable".) Similar for iOS project.

Reviewers: fpotter